LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 


OF" 


' 


ON    TWO    NEW 


Electrochemical     Processes 


FOR  THE 


Extraction  of  Silver  and  Gold 
from  Their  Ores 


THESIS 

Presented  in  Partial  Fulfillment  of  the  Requirements  for  the  Degree  ot 

Doctor  of  Philosophy  in  the  College  of  Chemistry 

of  the  University  of  California 

BY 

MOOSHEGH  VAYGOUNY 


BERKELEY,   1905 


ON    TWO    NEW 


Electrochemical     Processes 


FOR  THE 


Extraction  of  Silver  and  Gold 
from  Their  Ores 


THESIS 

Presented  in  Partial  Fulfillment  of  the  Requirements  for  the  Degree  ol 

Doctor  of  Philosophy  in  the  College  of  Chemistry 

of  the  University  of  California 

BY 

MOOSHEGH  YAYGOUNY 

fi 


BERKELEY,   1905 


CONTENTS 


Page 

Introductory 5 

Historical 6 

Theoretical 1 1 

I.     Chloridation  or  Sulphatation 14 

Experimental 21 

Electrolysis 32 

II.     Persulphatation 36 

Historical   36 

Theoretical 36 

Experimental 41 

Electrolysis 55 

Resume 59 

Acknowledgements * 61 


On  Two  New  Electrochemical  Processes 

for  the  Extraction  of  Silver  and 

Gold  from  Their  Ores 


INTRODUCTORY 

Some  two  years  ago,  Mr.  Gilbert  Gurney,  a  practical  mining 
man,  came  to  this  University  with  some  of  the  now  well- 
known  Tonopah  gold  and  silver  ores,  with  a  view  to  finding 
out  "what  could  be  done  with  them  in  the  line  of  a  remunera- 
tive treatment  of  the  same."  As  Mr.  Gurney,  upon  the  writer 
coming  to  know  him,  admitted  frankly  to  have  very  limited 
command  of  the  knowledge  of  chemistry  or  metallurgy  neces- 
sary to  c^pe  with  the  problem  in  question,  the  latter,  by  com- 
mon consent,  soon  devolved  upon  the  present  writer,  while 
Mr.  Gurney  himself  kindly  offered  to  purvey  certain  facilities 
whereby  the  necessary  investigations  were  to  be  carried  out. 

The  following  is  an  account  of  the  results  of  the  various 
experiments  and  studies  that  were  undertaken  in  'this  line 
during  the  academic  year  '03- '04,  and  continued  throughout 
the  year  '04- '05. 

It  is  necessary  to  state  here  that  nearly  all  the  assays  made 
in  connection  with  the  first  part  (Chloridation  method)  of  this 
study  were  made  by  Thomas  Price  &  Sons,  professional  as- 
sayers  of  San  Francisco,  through  the  courtesy  of  Mr.  Gurney. 
On  the  other  hand,  the  writer  assumes  responsibility  for  the 
remaining  assays,  all  of  which  were  made  at  the  chemical 
laboratories  of  this  University. 


t  *•?  Q  *  >  "i  > 


HISTORICAL 

It  seems  to  be  an  admitted  fact  in  mining  circles  that,  in 
spite  of  the  goodly  number  of  the  various  methods  proposed, 
some  in  journal  and  mostly  in  patent  literature,  in  the  last 
half  century  or  so,  the  desideratum  of  an  economic,  wet  process 
for  the  simultaneous  extraction  of  the  precious  metals  gold 
and  silver  from  their  sulphide  ores  is  still  keenly  felt.  In- 
deed, the  methods  in  vogue,  even  at  present,  are  still  some 
one  form  or  another  of  smelting,  amalgamation,  Augustin, 
or  hypo-sulphite  processes,  all  of  which  entail,  at  some  point 
or  other  in  the  course  of  their  operations,  certain  costly  treat- 
ments, now  in  the  form  of  a  preliminary  roasting,  and  now 
in  form  of  constant  consumption  or  rejection  of  the  reacting 
chemicals,  such  as  salt,  copper  sulphate,  and  mercury  or  its 
salts,  while  all,  except  smelting,  are  none  too  efficient,  even 
irrespective  of  the  cost  of  treatment. 

Although  at  the  time  the  writer  began  the  studies  which 
form  the  subject  of  this  paper,  and  for  some  time  later,  he  was 
quite  unaware  of  the  various  attempts  and  suggestions  made 
in  patent  literature  in  the  line  of  improved  methods  of  hand- 
ling such  precious  sulphide  ores,  and  though  such  patent  liter- 
ature, at  best,  could  constitute  anything  but  reliable  informa- 
tion to  guide  one's  steps  by,  it  is  none  the  less  of  interest  to 
first  give  here  a  cursory  account  of  the  most  important  meth- 
ods having  any  direct  bearing  on  those  to  be  advanced  in  these 
pages,  as  far  as  they  have  come  under  the  writer's  notice. 

While  the  use  of  purely  "wet"  or  chemical-reaction  methods 
had  been  introduced  as  early  as  the  sixteenth  century  by  the 
adoption  of  the  well-known  Patio  process — where  silver  sul- 
phides are  decomposed  by  means  of  a  solution  of  copper  sul- 
phate and  salt,  and  amalgamated — it  seems,  however,  that 
Becquerel*  was  the  first  to  realize  (in  1835)  the  importance 
of  the  introduction  of  some  neat  and  less  empirical  methods 


*Beoquerel,    Comptes    Rcndus,    vol.    38,    p.    109;    also,    Elements 
d'Electrochimie,  chap.  VI. 


in  the  process  of  silver  and  gold  ore  treatment.  Becquerel's 
idea  was  an  entirely  new  departure  in  the  metallurgical  meth- 
ods then  known.  He  suggested  to  first  oxidize  the  ores,  either 
by  a  roasting  or  by  means  of  a  solution  of  CuSO4  in  presence 
of  a  concentrated  solution  of  common  salt,  and  then  to  recover 
the  metals  thus  dissolved  by  using  voltaic  couples  of  zinc  and 
iron,  for  instance.  To  do  this,  the  less  electro-positive  metals 
were  dipped  in  the  solution  containing  the  metals  to  be  recov- 
ered, while  the  more  electro-positive  metals  were  immersed 
in  an  ordinary  solution  of  salt,  separated  from  the  first  by 
means  of  a  porous  membrane. 

Although  this  process  has  long  since  been  left  out  of  serious 
consideration,  and  for  obvious  reasons,  still,  if  we  remember 
that  not  only  was  this  method  proposed  in  those  early  days 
when  dynamos  were  unknown  things,  but  that  Becquerel  had 
the  patience  to  actually  treat  many  tons  of  ore  in  this  way, 
we  cannot  but  admire  the  insight  and  courage  of  that  great 
man. 

The  next  mention  of  an  improved  method  of  treating  sul- 
phide ores  is  that  of  Body,  who,  in  a  United  States  patent 
issued  in  1886,  proposed  to  use  a  solution  of  a  mixture  of 
ferric  salts  and  common  salt  as  the  agent  to  react  upon  the 
sulphides  in  general,  such  as  those  of  Ag,  Cu,  Pb,  etc.,  and 
the  electric  current  from  a  dynamo  machine  as  the  agent  to 
recover  the  dissolved  metals.  For  this  purpose  Body  advo- 
cated the  use  of  a  cement  tank  provided  with  two  quadrangu- 
lar compartments,  one  within  the  other,  and  separated  from 
each  other  by  means  of  a  porous  diaphragm.  Towards  the 
upper  part  of  the  cell  these  compartments  communicated  with 
each  other  freely,  as  the  diaphragms  did  not  stand  high  enough 
to  separate  them  completely.  The  inner  one  of  these  two 
compartments  was  provided  with  a  carbon  floor  serving  as 
anode,  while  the  cathodes  stood  upright  in  the  outer  com- 
partment. The  ore — previously  roasted  or  otherwise  oxidized, 
if  necessary — being  put  in  the  anode  or  inner  compartment, 
the  solution  was  admitted  from  below  and  made  to  ascend 
through  it,  while  the  entire  mass  was  being  kept  constantly 
stirred.  During  its  stay  in  this  compartment,  the  solution 
was  to  dissolve  the  desired  metals  by  acting  upon  their  sul- 
phides, thus  being  itself  reduced  to  a  solution  of  ferrous  salts, 


but  soon  oxidized  again  to  the  ferric  condition  by  virtue  of 
the  chlorine  here  generated  simultaneously  with  the  deposi- 
tion of  the  metals  on  the  cathodes  outside  under  the  influence 
of  the  electric  current.  To  facilitate  the  deposition  of  the 
metals,  the  solution  was  not  allowed  to  remain  long  in  this 
same  compartment,  but  was  continuously  forced  upward  and 
made  to  overflow  into  the  next  or  cathode  department,  and 
there  be  directly  electrolized  and  freed  from  its  metals.  This 
done,  the  solution,  which  must  not  only  have  now  been  im- 
poverished of  its  metals,  but  also  have  its  iron  salts  largely 
reduced  to  the  ferrous  condition,  was  next  returned  to  the 
anode  compartment  again,  there  to  have  first  oxidized  the 
latter  to  the  ferric  condition  and  then  to  have  resumed  the 
attack  of  the  remaining  sulphides. 

Needless  to  say  that,  in  spite  of  the  fact  that  this  process 
seems  to  have  been  widely  discussed  in  those  days,  it  does 
not  seem  to  have  as  yet  taken  a  practical  form  anywhere, 
and  for  obvious  reasons.  Indeed,  even  if  the  chemical  prin- 
ciples involved  left  nothing  to  be  desired,  the  idea  of  burying 
an  electrode  under  tons  of  non-conducting  ore-masses  could 
certainly  not  stand  the  test  of  any  practical  operation. 

In  the  same  year  that  Body's  patent  was  issued,  Endlich 
and  Muhlenberg*  secured  a  patent  claiming  the  use  of  "chlor- 
ine solutions  in  presence  of  other  metallic  chlorides,  such  as 
those  of  iron  and  copper,  with  or  without  the  addition  of 
common  salt."  These  inventors,  however,  did  not  specify 
by  what  means  they  intended  to  recover  the  metals  brought 
into  solution;  so  that  their  suggestion  does  not  even  stand 
the  test  of  a  fair  criticism. 

Soon  after  this,  also  in  the  same  year,  a  patent  was  issued 
to  Cassell,§  who  proposed  to  use  a  diaphragm  cell,  the  ores 
being  kept  in  the  anode  compartment,  somewhat  on  Body's 
principle,  but  using  merely  a  solution  of  common  salt  as  the 
lixiviating  agent.  It  was  claimed  that  during  the  passage 
of  an  electric  current  the  chlorine  generated  at  the  anode 
and  there  held  in  the  solution  impregnating  the  ore,  would 
act  upon  the  metallic  compounds  and  bring  about  their  attack 


*United  States  Patent  Office  Gazette,  October,   1886. 
^United  States  Patent  Office  Gazette,  October,   1886. 


and  dissolution.  This  done,  the  same  electric  current  was 
then  merely  to  carry  the  desired  metals  over  into  the  cathode 
compartment,  through  the  diaphragm,  and  there  deposit  them. 

It  will  be  seen  that  in  this  process  the  chief  defects  are, 
unlike  that  of  Body,  in  the  chemical  principles  involved  more 
than  in  practical  considerations,  since,  not  to  speak  of  other 
difficulties,  aquous  chlorine,  upon  whose  presence  the  oxida- 
tion of  the  sulphides  is  supposed  to  depend,  is  neither  an 
active  oxidant  of  such  sulphides  nor  can  it  be  got  in  any 
practically  useful  concentrations,  this  gas  being  only  slightly 
soluble  in  concentrated  brines. 

Some  two  years  later  there  appeared  the  patents  of  Dr. 
Hoepfner,*  whose  process  seems  to  have  caused  much  more 
excitement  in  certain  mining  circles  than  perhaps  any  of  the 
other  similar  electro-chemical  processes  advanced  before  or 
since  then.  Dr.  Hoepfner  proposed  to  use  solutions  of 
CuClL,  in  conjunction  with  common  salt  or  CaCL  and  their 
subsequent  electrolysis.  Although  his  chief  aim  was  to  treat 
ores  of  the  sulphides  of  copper,  his  process  was  also  expected 
to  extract  any  silver  and  nickel  which  may  accompany  the 
copper  in  the  ores. 

In  this  process  the  ores  were  finely  crushed  and  leached 
with  the  above  solution,  in  order  to  decompose  the  sulphides 
present  and  take  up  their  corresponding  metals.  The  solu- 
tion was  then  continuously  passed  through  special  diaphragm 
cells,  where  the  silver  was  either  to  be  deposited  on  special 
cathodes  by  means  of  a  current,  or  it  was  simply  to  be  pre- 
cipitated by  copper  shot.  This  done,  and  the  other  foreign 
metals  being  thrown  out  by  chemical  methods,  such  as  by  the 
use  of  CuO,  CaO,  KOH,  etc.,  the  solution  was  then  elec- 
trolyzed  with  a  proper  voltage  so  as  to  recover  the  excess  of 
copper  brought  into  solution  during  the  extraction  of  the  ore. 
During  this  latter  operation  the  remaining  cuprous  salts  were 
oxidized  to  cupric  salts,  and  could  therefore  be  used  over  again 
to  leach  the  ores  with. 

It  has  been  pointed  out  in  all  the  accounts  on  this  process — 
which  has  been  given  a  more  thoroughgoing,  practical  test 
than  perhaps  any  of  the  others,  though  without  success — that 


*Borchers,  Electric  Smelting  and  Refining. 

9 


one  of  the  principal  drawbacks  in  the  method  lies  in  the 
necessity  of  having  to  use  diaphragms.  Be  the  case  as  it 
may,  however,  it  only  stands  to  reason  that  such  a  process 
could  not  be  used  for  exclusively  silver  ores  for  reasons  eco- 
nomical and  practical,  as  will  be  indicated  further  on. 

Besides  the  above-enumerated  more  prominent  methods,  the 
writer  has,  through  the  kindness  of  Dr.  W.  J.  Sharwood, 
been  able  to  come  to  know  of  a  list  of  not  less  than  fifteen  to 
twenty  different  patents,  issued  in  this  country  or  abroad, 
all  relating  to  the  extraction  of  silver  and  gold  by  the  use 
of  solutions  of  copper  or  iron  salts,  now  in  the  form  of  sul- 
phates, now  as  chlorides;  sometimes  in  conjunction  with  the 
use  of  a  strong  solution  of  common  salt,  sometimes  without 
it ;  now  by  introducing  some  free  acids,  and  now  none,  while 
nearly  all  prescribe  a  preliminary  roasting  and  some  form  or 
other  of  a  diaphragm  cell  whenever  electrical  methods  are 
explicitly  stated  to  be  employed  in  recovering  the  metals 
brought  into  solution. 

Such  was  found  to  be  the  status  of  the  problem  of  the 
chemical,  wet  methods  of  extracting  the  precious  metals  from 
their  ores  when  the  writer  first  took  cognizance  of  them. 
It  needs  no  special  argument  to  show  that,  in  face  of  such 
accounts  of  "suggested,"  but  "unhatched,"  "patent"  methods  of 
various  degrees  of  theoretical  soundness,  but  of  no  positive 
reliability,  the  problem  could  still  be  fairly  considered  one 
awaiting  a  special  study,  and  a  solution,  if  possible.  In- 
deed, it  is  a  recognized  fact  that  not  a  few  of  the  patents 
granted  every  year  in  such  chemical  or  metallurgical  lines 
represent  mere  hastily  conceived  ideas,  unbacked  by  any  sound 
experimental  data,  and  that  the  few  methods  which  have  found 
their  way  into  text-books  can  be  called  anything  but  of  un- 
questionable authenticity,  they  being  more  or  less  the  results 
of  hearsay  or  gathered  from  patent  accounts. 

Owing  to  the  above  considerations,  it  will  therefore  not 
be  improper,  in  the  following  pages,  to  carry  on  the  discus- 
sion entirely  without  reference  to  these  patent  suggestions. 
This  will  be  the  more  logical  as  the  first  and  essential  steps 
in  these  studies  had  been  taken,  as  was  mentioned  above, 
long  before  the  writer  was  aware  of  the  existence  of  the 
methods  sketched  or  alluded  to. 

10 


THEORETICAL 

On  first  venturing  to  develop  any  chemico-metallurgical 
method  of  treating  ores  of  the  type  here  under  consideration, 
the  first  points  to  be  considered  are,  naturally,  relative  to  the 
requirements  to  be  fulfilled  by  the  process  to  be  worked  out. 
In  the  case  on  hand,  what  are  these  requirements?  They 
are  obviously  that  the  method  should  be  economical ;  it  should 
be  a  lixiviation  process  pure  and  simple,  as  far  as  possible; 
and,  if  the  method  is  at  all  to  be  of  wide  applicability,  it 
should  be  capable  of  extracting  both  the  metals  in  question- 
silver  and  gold — as  they  occur  so  frequently  together.  Fur- 
ther, it  must  do  away  with  any  preliminary  roasting,  which 
is  more  or  less  troublesome  and  expensive.  On  the  other 
hand,  the  solution  used  must  neither  be  costly  nor  have  any 
properties  deleterious  to  health ;  the  metals  dissolved  thereby 
must  admit  of  being  readily  recovered ;  and,  finally,  the  solu- 
tion should  lend  itself  to  being  regenerated  by  some  appro- 
priate and  ready  means,  so  as  to  avoid  the  necessity  of  its 
being  constantly  renewed ;  in  other  words,  the  method  should 
be  "cyclic." 

Now,  given  the  fact  that  in  the  entire  field  of  chemistry 
there  is  no  solution  of  a  known  compound  that  will  dissolve 
the  sulphides  in  question,  as  such,  sufficiently  and  economically, 
without  the  necessary  oxidation  of  the  latter,  it  becomes 
forced  upon  the  investigator — in  view  of  the  above  condition- 
ing factor  that  all  roasting  operations  should  be  avoided,  as 
far  as  possible — that  he  choose  only  such  solutions  as  have 
naturally  oxidizing  properties.  But  all  oxidation  reactions 
involve  concomitantly  a  reduction  on  part  of  the  oxidizing 
agents  which  may  be  employed;  hence,  if  these  agents  are  to 
be  used  cyclically,  they  must  necessarily  admit  of  being  read- 
ily oxidized  back  into  their  original  conditions.  Further, 
since  economic  operation  requires  that  there  should  be  no 
decomposition  products  arising,  not  only  during  the  attack 
of  the  sulphides,  but  also  during  such  regenerating  operations, 
the  compound  or  compounds  to  be  selected  as  the  solvent  agent 
desired  must  be  fairly  stable  and  be  very  "labile,"  or  capable 
of  taking  up  first  a  higher,  then  a  lower  stage  of  oxidation. 
Then,  too,  since  the  best  and  neatest  method  of  recovering 

ii 


the  metals  dissolved  would  be,  beyond  doubt,  one  depending 
upon  the  use  of  the  electric  current,  it  would  be  highly  desir- 
able that  the  compounds  selected  should  be  capable  of  being 
oxidized  by  means  of  this  same  current,  best  concurrently 
with  the  electro-deposition  of  the  metals. 

If  now,  with  these  data  on  hand,  we  inquire  as  to  which 
compound  or  compounds  will  meet  the  requirements  imposed, 
we  will  notice  that  there  are  only  two  groups  of  salts  that 
will  answer  for  the  purpose  at  issue,  and  which  are  at  the 
same  time  sufficiently  cheap  and  available.  We  have: 

I.       THK    SULPHATES    AND    THE    CHLORIDES 

of  those  common  elements,  with  varying  valencies  or  degrees 
of  oxidation,  as  are  also  sufficiently  soluble  in  water,  namely, 
those  of  iron,  copper,  and  manganese. 

II.       THE    SO-CALLED    PERSULPHATES 

A  comparative  view  of  the  desirabilities  of  the  salts  of  iron, 
copper,  and  manganese  will,  however,  show  that  not  all  these 
three  classes  of  bodies  are  equally  desirable.  For  while  a 
moderately  acid  solution  of  the  above  iron  salts  may  well  be 
electrolyzed  without  any  of  the  metal  separating  at  the  elec- 
trodes, those  of  copper  and  manganese,  on  the  other  hand, 
cannot  be  so  treated  with  advantage.  For,  with  solutions 
of  copper  salts,  not  only  would  we  have  economic  difficulties 
to  meet  with  in  the  line  of  the  large  amounts  of  copper 
which  would  have  to  be  locked  up  in  the  stock  solutions  to 
be  used,  but,  what  is  more,  such  solutions  could  only  be  freed 
from  undesirable  base  metals,  such  as  Sb,  As,  etc.,  which 
would  accumulate  in  them  and  foul  them  in  time,  with  both 
complicated  and  costly  chemical  methods  of  precipitation  of 
these  impurities.  This  would  be  the  more  unavoidable,  as 
it  would  certainly  be  difficult  to  avail  one's  self  of  the  neater 
methods  of  simple  electrolysis  with  higher  voltages  to  remove 
these  base  metals,  lest,  in  doing  so,  copper  should  also  be 
deposited  on  the  cathodes,  and  thereby  cause  much  trouble. 
Again,  with  the  salts  of  manganese,  the  oxide  MnO2  is  likely 
to  appear  at  one  or  both  electrodes  during  the  electrolytic 
winning  of  the  metals,  and  thus  give  rise  to  many  unnecessary 
difficulties. 

12 


It  thus  becomes  clear  that,  of  the  salts  of  the  above  three 
commoner  elements,  those  of  iron,  when  used  in  conjunction 
with  some  free  acids,  are  much  to  be  preferred,  since,  besides 
presenting  none  of  the  above  difficulties,  they  are  also  the 
cheapest  and  most  readily  got. 

Leaving  the  study  of  the  persulphates  for  the  second  part 
of  this  paper,  we  may  then  first  consider  here  the  application 
of  the  iron  salts  to  the  extraction  of  the  precious  metals. 


13 


I.     CHLORIDATION   OR  SULPHA- 
TATION 


If  we  attempt  to  study,  in  a  preliminary  way,  the  relative 
adaptations  of  the  chloride  and  the  sulphate  of  iron  by  carry- 
ing out  some  qualitative  experiments  with  artificially  prepared 
silver  sulphide,  and  with  two  separate  solutions,  one  made 
up  of  about  i  to  2%  FeCl3  and  HC1,  the  other  consisting  of 
about  the  same  amount  of  Fe2(SO4)3  and  H2SO4,  we  ob- 
serve that,  so  far  as  their  efficiencies  in  oxidizing  the  silver 
sulphide  is  concerned,  whether  in  the  cold  or  in  the  warm, 
there  is  no  appreciable  difference  to  be  noticed  in  favor  of 
one  or  the  other  of  these  solutions. 

Now,  seeing  that  it  is  a  matter  of  no  small  economic  im- 
portance whether  one  deals,  in  practice,  with  sulphates  or 
with  chlorides,  it  might  seem  here,  a  priori,  that  a  sulphuric 
acid  solution  of  ferric  sulphate  should  be  preferable  to  one  of 
HC1  and  FeCl3.  This  would  be  especially  so,  inasmuch  as, 
in  practice,  such  a  solution  would  be  expected  to  form  the 
more  soluble  Ag2SO4,  instead  of  the  insoluble  AgCl,  on  re- 
acting with  the  sulphides  of  this  metal,  and  that,  therefore, 
it  would  lend  itself  better  to  the  subsequent  leaching  out  of 
the  values  from  the  ore  masses. 

Such  a  choice,  however,  would  be  objectionable  for  two 
main  reasons.  In  the  first  place,  Fe2(SO4)3  having  no 
appreciable  solvent  action  on  free  gold,  its  exclusive  use  would 
not  be  in  conformity  with  the  desideratum  set  previously, 
namely,  the  simultaneous  extraction,  as  far  as  possible,  of 
both  the  silver  and  the  gold  contained  in  ores.  In  the  second 
place,  the  use  of  such  simple  sulphate  solutions  would  be 
amiss,  even  with  ores  that  contain  no  appreciable  amounts 
of  gold  but  are  more  exclusively  silver  ores,  because,  there 
being  no  natural  ore,  nor  a  method  of  ore  treatment  possible 
that  could  avoid  sufficiently  the  introduction  of  some  chlorine 
compounds  during  such  treatments,  there  would  always  be 
a  liability  for  the  conversion  into  AgCl  of  much  of  the  dis- 

14 


solved  Ag2SO4  formed  under  the  oxidizing  influence  of  the 
ferric  sulphate  upon  the  silver  sulphides.  In  fact,  such  a 
change  would  be  undesirable  not  merely  owing  to  the  im- 
practicability thus  arising  of  the  removal  of  the  silver  values 
by  a  simple  leaching,  but  also  because  the  formation  of  the 
insoluble  AgCl  would  tend  to  hinder  the  complete  oxidation 
of  the  silver  sulphides,  owing  to  a  protective  coating  which 
this  compound  would  form  and  envelop  the  particles  of  the 
sulphides  unless  it  be  dissolved  away  and  removed  as  fast  as 
it  is  formed. 

If,  therefore,  ferric  sulphate  is  at  all  to  be  chosen  as  the 
oxidizing  agent  desired,  it  is  essential  to  use  concurrently 
with  it,  on  the  one  hand,  some  simple,  appropriate  solvent 
of  AgCl,  and,  on  the  other,  one  of  free  gold. 

Unfortunately,  of  all  the  simpler  solvents  of  AgCl  or  of 
gold,  there  seems  to  be  none  known  capable  of  co-existing 
in  solution  with  ferric  salts,  let  alone  the  consideration  of  their 
cheapness.  Thus,  cyanides  could  not  be  used  because  of  the 
liability  of  their  giving  rise  to  insoluble  ferro  or  ferri  cyanides, 
thus  resulting  in  loss  and  in  general  nuisances.  On  the  other 
hand,  thiosulphates  and  bisulphites  could  not  be  used  owing 
to  their  being  decomposed  in  presence  of  ferric  salts ;  nor,  for 
obvious  reasons,  could  NH4OH  be  used  for  the  purpose  under 
consideration. 

There  is,  however,  but  one  possible  solution  to  this  problem, 
and  that  lies  in  the  use  of  high  concentrations  of  some  suitable, 
soluble  chloride,  such  as  common  salt,  in  conjunction  with 
these  sulphates.  Such  chloride  solutions,  indeed,  are  especially 
appropriate,  as  by  their  use  not  only  is  it  possible  to  bring 
about  the  solution  of  gold  readily  under  the  influence  of  the 
free  chlorine  which  can  be  generated  and  made  to  saturate 
the  main  solution  simultaneously  with  the  electro-deposition 
of  the  dissolved  metals,  but  particularly  because  they  also 
afford  a  ready  ^means  for  the  dissolution  and  removal  of  the 
silver  values  concurrently  with  the  gold.  It  seems,  indeed, 
a  peculiarity  of  silver  chloride  that  it  will  go  into  solution 
quite  appreciably  in  presence  of  other  chlorides,  especially 
those  of  the  alkali  and  alkaline-earth  metals,  and  more  par- 
ticularly in  free  HC1.  No  doubt  the  secret  and  the  sole  func- 
tion of  the  use  of  such  large  amounts  of  common  salt  in  con- 


junction  with  CuSO4  in  the  old  Patio  amalgamation  process 
lies  in  just  this  effect  of  the  salt  being  capable  of  holding 
in  solution  the  silver  chloride  which  is  formed  under  the 
oxidizing  effect  of  the  CuSO4  on  the  sulphides  of  this  metal, 
so  that  the  mercury  may  with  readiness  first  reduce  this  latter 
from  its  solution  to  the  metallic  state  and  then  amalgamate 
with  it. 

But  now,  if  such  high  concentrations  of  chlorides  are  to  be 
used,  as  they  needs  must,  with  a  solution  of  ferric  sulphate 
and  sulphuric  acid,  it  at  once  becomes  evident  that  there  re- 
mains no  serious  advantage  in  favor  of  such  a  solution  over 
one  of  ferric  chloride  and  either  HC1  or  H2SO4.  Indeed, 
so  far  as  economic  differences  in  the  use  of  one  or  the  other  of 
these  compounds  are  concerned,  none  can  exist  worth  being 
called  serious,  inasmuch  as,  once  the  necessary  supply  of 
either  salt  is  secured  and  the  stock  solution  made  up,  the 
process  being  cyclic,  the  concentration  of  the  iron  in  solution 
will  not  diminish  sufficiently  to  necessitate  any  continued  sup- 
ply of  such  salts,  if,  indeed,  it  be  ever  necessary  to  buy  these, 
even  in  long-continued  practice,  rather  than  depend  upon  the 
iron  contents  of  the  ores  themselves  to  furnish  what  little 
amount  of  iron  salt  be  requisite  at  any  time. 

On  the  other  hand,  though  there  is  certainly  an  apparently 
decided  difference  in  the  cost  of  the  two  acids  HC1  and  H2SO4, 
still,  it  being  true  that  to  whatever  extent  SO4  radicals  are 
substituted  by  Cl  radicals  to  that  extent  will  the  solvent  power 
of  the  solution  on  AgCl  be  enhanced — a  fact  which  alone  is 
of  nature  to  counterbalance  what  slight  difference  there  may 
be  in  original  outlays  in  securing  these  acids — even  the  use 
of  one  or  the  other  of  these  compounds  becomes  more  or  less 
a  matter  of  choice  in  a  cyclic  process  like  that  in  question. 
Indeed,  should  even  a  given  ore  be  too  alkaline  to  stand  the 
cost  of  the  HC1  necessary  to  first  neutralize  it,  it  may  be  treated 
with  H2SO4  in  a  preliminary  way ;  or  one  may  even  use  HC1 
itself  in  such  extreme  cases,  allowing  this  acid  to  percolate 
through  the  mass,  and  subsequently  treating  the  collected 
solutions — now  containing  all  the  calcium  as  CaCL — with 
H2SO4,  thus  rejecting  this  metal  as  CaSO4  by  a  simple  filtra- 
tion and  at  the  same  time  regenerating  the  HC1  which  had 
combined  with  the  calcium. 

16 


On  the  whole,  therefore,  it  seems  quite  immaterial,  for 
the  purpose  on  hand,  whether  one  uses  a  solution  made  up 
of  Fe2(SO4):!  and  H2SO4  in  presence  of  a  cheap  chloride, 
such  as  NaCl,  or  whether  one  chooses  a  solution  of  FeCl3 
and  HC1  or  H2SO4,  with  the  same  cheap  salt,  or  of  CaCL, 
to  furnish  the  necessary  concentration  of  chlorine  radicals. 
As  for  the  concentration  of  the  reagent  solution  in  these 
various  compounds,  one  need  not  diverge  materially  from  the 
approximate  composition  given  on  a  previous  page,  though 
the  concentration  of  the  common  salt  (NaCl)  or  of  CaCl2 
may  best  be  made  as  high  as  20%. 

But  it  may  be  objected  here  that  such  strong  solutions  as 
the  above  may  not  be  the  best  that  could  be  wished  from  the 
standpoint  of  facilities  in  commercial  handling.  However, 
considering  the  fact  that  these  solutions  can  be  cheaply  got, 
readily  recovered,  and  regenerated  indefinitely,  coupled  with 
the  fact  that  they  possess  high  electric  conductivities,  and 
therefore  require  less  expenditure  of  power  for  the  electro- 
lytic work,  than  were  they  more  dilute,  these  facts  are  certainly 
of  a  nature  to  more  than  counterbalance  what  little  disad- 
vantage this  high  concentration  ma-y  offer. 

Reactions  Involved. — Before  giving  quantitative  data  to 
show  to  what  extent  the  use  of  ferric  salts,  under  proper  con- 
ditions, can  be  regarded  satisfactory  for  the  extraction  of  the 
precious  metals  from  their  ores,  it  is  of  interest  to  consider 
here  first  the  mechanisms  of  the  reactions  involved  in  their  use. 

In  carrying  out  qualitative  experiments  with  the  above  solu- 
tions under  the  proper  conditions,  namely,  in  presence  of 
much  NaCl,  for  instance,  we  observe,  especially  if  the  solutions 
be  warmed,  that  the  Ag2S  is  rapidly  decomposed.  At  the 
same  time  silver  goes  into  solution  and  much  of  the  ferric 
iron  present  is  reduced  to  the  ferrous  condition,  while  a  gray- 
ish scum  of  sulphur  appears  floating  on  the  surface  of  the 
solution.  The  reaction  taking  place  seems  therefore  to  be, 
with  the  sulphate  of  iron : 

Ag|hFe2(S04),  =  AgaS04+2Fe.S04+Sj( 
Ag2S04+2NaCl  ==  2AgCl+Na2SO4 

and  with  the  chloride  of  iron: 

Ag2S+2FeCl3+NaCl  =  2AgCl-f  2FeCl2+S-fNaCl 

17 


Now  while  this  seems  to  be  the  simplest  way  for  represent- 
ing the  facts  observed — and  it  is  indeed  the  one  commonly 
accepted — still  it  is  a  question  whether  the  reaction  involved 
is  as  direct  as  that  shown  above.  For  the  same  facts  may 
be  represented  equally  as  well  by  the  following  equations, 
considering  exclusively  chloride  solutions  alone  for  clearness' 
sake: 

Ag2S+2HCl  =  2AgCl+H2S 
and  then :  n 

H2S+2FeCl3  =  2HCl^FeCl,+  S 

The  mere  fact  that,  ordinarily,  acids  alone  have  no  appre- 
ciable effect  upon  the  sulphide  of  silver  is  certainly  no  cri- 
terion against  such  a  view.  For  if  this  preceding  reaction 
be  regarded  as  a  reversible  equilibrium  reaction — as  all  re- 
actions may  be  so  looked  upon — only  with  its  equilibrium  point 
towards  the  extreme  left,  namely,  toward  the  formation  of 
Ag2S,  it  becomes  clear  that  if  we  only  remove  the  disturbing 
reaction  product,  H2S,  by  some  appropriate  method,  the  point 
of  equilibrium  could  be  readily  shifted  to  the  right,  so  that 
AgCl  could  form  freely.  The  function  of  the  ferric  salt 
would  therefore  be,  in  this  light,  only  a  secondary  one,  namely, 
one  of  removing  this  disturbing  reaction  product,  H2S,  as 
fast  as  it  is  formed  under  the  direct  influence  of  the  acids. 
That  this  view  is  more  than  a  mere  supposition  is  evidenced 
by  the  fact  that  when  a  mixture  of  Ag2S  and  HC1,  for  in- 
stance, is  boiled,  even  in  absence  of  any  oxidizing  agents, 
there  is  a  slight  decomposition  of  the  sulphide  taking  place, 
especially  if  the  acid  used  be  concentrated.  It  being  permissi- 
ble to  regard  the  effect  of  boiling  as  merely  facilitating  the 
expulsion  of  H2S,  this  phenomenon  may  well  be  taken  as  an 
indirect  evidence  in  favor  of  the  above  view.  The  very  fact, 
indeed,  that,  even  in  commercial  practice,  Ag2S  is  decomposed 
by  concentrated  H2SO4  in  the  absence  of  oxidants  may  here 
be  adduced  as  even  a  more  tangible  evidence. 

Again,  the  fact  that  a  solution  of  a  ferric  salt,  free  from 
any  surplus  foreign  acids,  decomposes  Ag2S  cannot  be  ad- 
duced as  necessarily  favoring  the  existence  of  a  direct  action 
between  these  two  compounds,  inasmuch  as  all  ferric  salts  are 
hydrolyzed  freely,  and  are  therefore  apt  to  play  the  role  both 
of  acids  and  of  oxidizing  agents. 

18 


In  view  of  these  considerations,  it  seems  that  the  reactions 
whereby  the  oxidation  of  Ag.,8  takes  place  may  best  be  ex- 
plained in  the  light  of  the  second  view,  unless,  indeed,  it  be 
admitted  that  both  the  direct  and  the  indirect  reactions  take 
place  concurrently. 

As  regards  the  mechanism  whereby  the  AgCl  goes  into  so- 
lution in  presence  of  the  foreign  chlorides,  there  seems  to  be 
conclusive  evidence  that  this  takes  place  by  virtue  of  a  ten- 
dency the  former  compound  has  to  give  rise,  with  these 
chlorides,  to  double  salts  analogous  to  double  iodides  and 
double  cyanides.  Such  a  combination  of  AgCl  and  NaCl  is 
readily  obtained  in  crystalline  form  whenever  the  conditions 
are  such  that  NaCl  will  begin  to  separate,  such  as  by  concen- 
trating a  solution  containing  these  two  compounds,  by  evapo- 
ration, or  by  adding  concentrated  acids  to  it,  especially  HClr 
In  either  case,  the  first  crop  of  crystals  that  separate  will 
contain  practically  all  the  silver  chloride.  These  crystals  are 
visibly  cubic  and  look  exactly  like  so  many  crystals  of  pure 
NaCl,  and,  strange  enough,  they  do  not  even  seem  to  be 
affected  by  light.  They  are,  however,  readily  decomposed 
into  flocculent  AgCl  and  NaCl  when  thrown  into  pure  water. 

Unfortunately  it  is  very  difficult  to  form  a  definite  idea  as 
to  the  exact  composition  of  the  double  salt  represented  in  these 
crystals,  as  they  do  not  seem  to  be  of  any  fixed  composition, 
but  rather  individuals  produced  by  the  simultaneous  and  iso- 
morphous  crystallization  of  both  a  double  salt  and  of  inde- 
pendent molecules  of  NaCl.  Although  treatises  on  chemistry* 
do  not  seem  to  make  specific  mention  of  a  double  salt  of  silver 
and  sodium  chlorides,  still,  as  they  allude  in  a  general  way 
that  AgCl  can  form  double  salts  of  the  composition  AgClMCl, 
it  seems  that  the  compound  here  under  consideration  may  best 
be  represented  as 

AgCl.NaCl+.rNaCl 

If  this  be  correct,  and  it  seems  to  be  strengthened  by  the 
fact  that  several  attempts  to  obtain  this  compound  in  a  form 
sufficiently  free  from  extra  NaCl  for  analysis  have  resulted 
in  failure,  then  the  salt  here  under  consideration  would  rep- 


*Watt's    Dictionary    of    Chemistry    (Muir    and    Morley)  ;    Laden- 
burg's  Handivoerterbuch  der  Chcmie. 

19 


resent  a  definite  case  of  crystallization  where  double  salt  for- 
mation and  isomorphism  may  coexist. 

There  remains  now  to  consider  the  mechanism  of  the  oxi- 
dation of  gold. 

It  must  be  admitted  here  that,  in  spite  of  the  assertions  of 
certain  authorities,*  no  appreciable  solution  of  gold  can  take 
place  in  solutions  of  ferric  iron,  whether  in  form  of  chloride 
or  sulphate,  except  in  presence  of  some  foreign  oxidizing 
agent,  such  as  chlorine,  nitrates,  and  even  ordinary  atmos- 
pheric oxygen. §  This  is  certainly  not  surprising,  seeing  that 
such  an  attack  of  gold  by  a  ferric  salt  would  give  rise  con- 
comitantly  to  so  much  of  a  ferrous  salt,  and  that  this  latter 
would  naturally  tend  to  react  backwards  and  cause  the  re- 
precipitation  of  gold  by  virtue  of  the  well-known  effects  of 
ferrous  salts  on  solutions  of  gold. 

It  is  evident,  therefore,  that  the  essential  condition  for  the 
successful  attack  of  free  gold,  along  with  the  oxidation  of  the 
sulphides  of  silver  and  other  metals,  lies  in  the  removal  of 
the  ferrous  salts  formed  during  such  reactions,  as  represented 
on  a  previous  page. 

Such  an  oxidation  of  ferrous  salts  can  of  course  be  brought 
about  best  by  means  of  the  chlorine  which  will  tend  to  be 
liberated  at  the  anodes  during  the  electro-deposition  of  metals, 
and  with  which  the  main  solution  may  be  made  to  be  saturated 
in  actual  practice.  The  reactions  here  involved  would  then  be: 

Fed.,  +  Cl  =  FeCl3 
3FeS04  +  Cl,  =  Fe2(S04)8  +  Fed, 

in  both  of  which  cases  the  ferric  salts  regenerated  can  then 
react  upon  gold  to  the  extent  that  there  is  free  chlorine  left  on 
hand  in  the  solution,  and  on  the  sulphides  to  the  extent  that 
there  is  any  iron  left  in  the  ferric  condition. 

Now  a  critical  consideration  of  the  above  reactions  involved 
in  this  alternate  oxidation  and  reduction  of  the  iron  salts  will 
reveal  the  fact  that  while  in  purely  chloride  solutions  the 
iron  will  always  remain  combined  with  chlorine — that  is,  it 
will  be  present  as  chloride — with  solutions,  on  the  other  hand, 


*Comey's  Dictionary. 

IMcIlhiney,  Am.  Jour,  of  Science,  '96,  vol.  ^"152,  p.  293. 

20 


where  the  iron  may  have  been  originally  introduced  as  a 
sulphate  there  will  be  a  continuous  tendency  for  them  to 
have  this  sulphate  of  iron  converted  into  the  chloride  form. 
It  is,  indeed,  clear  that  every  time  Fe2(SO4)8  is  reduced  to 
FeSO4,  the  subsequent  oxidation  of  this  latter  by  means  of 
chlorine  will  convert  part  of  the  iron  present  into  FeCl:?,  as 
is  shown  in  the  second  reaction  above ;  so  that  when  this 
process  is  repeated  many  times  there  will  be  more  and  more 
sulphate  of  iron  disappearing,  while  chloride  of  iron  will  take 
its  place. 

Owing  to  this  consideration,  it  is  important  to  note  here 
that  the  method  of  ore  treatment  here  discussed  should  be 
properly  regarded  as  a  "chloridation"  method,  at  least  for 
clearness'  sake,  even  though  the  iron  originally  introduced 
may  have  been  in  the  sulphate  form,  or  the  acid  in  the 
stock  solution  may  be  H2SO4,  and  not  HC1. 

We  may  now  pass  to  a  quantitative  study  of  the  extent  of 
suitability  of  such  chloride  solutions  of  iron  salts  to  the  treat- 
ment of  some  actual  ores  and  the  conditions  of  operation. 


EXPERIMENTAL 

EXPERIMENTS  WITH  ORE  NO.    I 

In  the  following  experiments  the  ore  used  was  obtained 
from  Tonopah,  Nevada.  Ore  No.  I  was  a  grayish  quartzose 
ore,  containing  some  galena,  pyrite,  calcite,  silver  sulphides, 
and  gold,  with  a  gangue  consisting  mainly  of  quartz.  It  also 
contained  a  notable  proportion  of  metallic  iron,  introduced 
into  the  ore  owing  to  improper  grinding. 

Experiment  I. — In  'this  experiment  the  ore  was  ground  to 
80  mesh  to  start  with,  owing  to  its  reputation  of  being  very 
refractory,  and,  instead  of  treating  it  with  a  stock  solution 
already  made  up,  it  was  thought  best  to  utilize  the  metallic 
iron  in  the  ore  by  dissolving  it  in  an  acid.  As  it  was  of 
importance  to  first  study  the  effect  of  sulphate  solutions,  the 
acid  chosen  here  was  H2SO4.  When  the  action  had  ceased, 
an  excess  of  this  acid  and  NaCl  was  added,  so  that  the  result- 
ing solution  contained  about  .5%  metallic  iron,  2%  H2SO4, 
and  20%  NaCl,  with  a  total  volume  of  I  liter  for  an  ore  mass 

21 


of  I  kilo.  The  mixture  was  put  into  a  large  flask,  chlorine 
was  introduced  into  it  so  as  to  oxidize  the  FeSO4  formed 
under  the  influence  of  the  H2SO4  upon  the  metallic  iron  in 
the  ore,  and  a  mixture  of  ferric  sulphate  and  chloride  was 
thus  obtained  in  the  ratio  of  5  \2. 

In  order  to  facilitate  the  reactions  and  shorten  the  time  of 
treatment,  heat  was  then  applied  to  the  mass,  which  was  kept 
nearly  boiling  for  three  hours,  care  being  taken  that  there 
was  always  an  abundance  of  chlorine  on  hand  in  the  solution 
throughout  this  period.  At  the  end  of  this  time  the  mass 
was  filtered  and  well  washed  with  a  strong  solution  of  com- 
mon salt  until  a  few  drops  of  fresh  teachings  showed  no 
turbidity  on  being  diluted  with  water,  which  meant,  of  course, 
that  all  the  silver  chloride  held  in  the  solution  wetting  the 
ore  mass  had'  been  washed  out.  (It  is  hardly  necessary  to 
say  here  that  it  was  essential  to  use  a  concentrated  solution  of 
salt  for  this  operation,  because,  had  ordinary  water  or  even 
a  very  dilute  solution  of  salt  been  used,  the  liquors'  impreg- 
nating the  ore  mass  would  be  diluted  thereby,  and  thus  much 
silver  chloride  might  be  precipitated  and  left  back  in  the  ore.) 

When  this  was  done,  the  mass  was  further  washed  with 
pure  water  to  remove  the  excess  of  salt  solution  wetting  it, 
and  lastly  it  was  dried  and  sent  to  professional  assayers  as 
tailings. 

The  following  are  the  results  of  the  assays  of  the  sample 
before  and  after  treatment : 

RESULTS    OF    ASSAY,    EXPERIMENT    I.      ORE    NO.     I 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Original  .8  84.7 

Tailings  .35  .  4.8 

This  corresponds  to  about :       56%      extraction  of  gold 
and      944%  extraction  of  silver. 

It  will  be  seen  from  these  data  that  there  was  much  room 
for  improvement  as  regards  the  extraction  of  the  gold  values. 

Having,  however,  obtained  similar  results  in  a  second  test, 
under  similar  conditions,  it  was  then  thought  interesting  to 
study  the  effect  of  the  finer  crushing  of  the  ore  upon  the 
extraction  of  the  values. 

22 


Experiment  II. — Another  lot  of  the  same  ore  was  therefore 
crushed  to  120  mesh  and  assayed  for  the  second  time.  The 
treatment  carried  out  in  this  experiment  was  identical  to  the 
above  in  details  as  to  the  composition  of  solution,  length  of 
time  of  boiling  or  treatment,  and,  of  course,  as  to  care  in  the 
method  of  washing  the  tailings.  Only  it  was  carried  out  on 
a  lot  of  1/2  pound,  with  the  corresponding  amount  of  solution, 
instead  of  I  kilo,  of  ore,  as  was  done  in  the  first  experiment. 

RESULTS   OF   ASSAY.       EXPERIMENT    II.       ORE    NO.     I 

Aii.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Original  1.03  86.2 

Tailings  .125  i.i 

This  corresponds  to  about:     88%     extraction  of  gold 
and       98.7%  extraction  of  silver. 

It  will  now  be  noticed,  on  comparing  the  results  of  these 
two  experiments,  that  a  good  deal  of  the  gold  values  of  this 
ore  are  very  closely  locked  up  in  the  ore  gangue,  thus  necessi- 
tating very  fine  crushing,  and  that,  so  far  as  the  silver  values 
are  concerned,  So-mesh  crushing  is  practically  as  good  as 
I2o-mesh. 

Experiment  III. — Since  in  the  above  experiments  the  solu- 
tion had  to  be  kept  boiling  during  the  treatment,  and  since 
the  keeping  warm  of  a  mass  of  ore  is  quite  an  item  in  large- 
scale  practice,  the  necessity  of  some  experiments  to  study  the 
effect  of  treatment  in  the  cold  became  now  quite  evident. 
The  following  test  was  therefore  carried  out  to  elucidate  this 
point. 

The  ore  treated  was  y%  pound  of  the  I2o-mesh  sample.  The 
details  of  treatment  were  identical  to  those  in  the  above  tests, 
except  that  they  were  carried  out  in  the  cold,  and  the  time  of 
treatment  was  prolonged  to  two  days,  with  occasional  shaking. 

RESULTS   OF   ASSAY.       EXPERIMENT    III.       ORE    NO.    I 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Original  .  1.03  86.2 

Tailings  .575  18.7 

This  corresponds  to:       44%  extraction  of  the  gold 
and      79%  extraction  of  the  silver. 

23 


These  data  clearly  showed  the  necessity  of  further  prolonga- 
tion of  time  of  cold  treatment,  and  consequently  the  following 
test  was  then  made : 

Experiment  IV. — The  conditions  in  this  experiment,  too, 
were  identical  to  those  of  the  preceding  experiment,  except 
that  hydrochloric  acid,  instead  of  sulphuric,  was  used  to  dis- 
solve the  free  iron  in  the  ore  mass,  whereby  all  the  iron  was 
put  in  the  chloride  form  on  chlorinating  it,  instead  of  part  of 
it  staying  in  the  sulphate  condition.  The  time  of  treatment 
was  extended  to  three  days  instead  of  two,  as  in  the  preceding 
test. 

RESULTS   OF  ASSAY.       EXPERIMENT   iy.      ORE    NO.    I 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Original  1.03  86.2 

Tailings  .325  6.6 

This  corresponds  to:       69%      extraction  of  gold 
and       92.4%  extraction  of  silver. 

From  these  results  it  can  be  seen  that  a  three  days'  treatment 
of  the  ore  with  a  cold  solution  will  extract  highly  satisfactory 
percentages  of  silver  values. 

It  might  be  argued  here  that  the  favorable  difference  ob- 
tained by  this  last  test  might  have  been  due  not  to  the  pro- 
longation of  the  time  of  treatment,  but  rather  to  the  fact  that 
in  using  hydrochloric  acid  instead  of  sulphuric,  more  ferric 
chloride,  and  therefore  more  chlorides,  were  introduced  into  the 
solution  than  in  the  first  case. 

Experiment  V . — In  order  to  determine  to  what  extent  such 
may  have  been  the  case,  a  special  experiment  was  carried  out 
subsequently  under  the  same  conditions  as  with  the  preceding 
test,  only  substituting  H2SO4  for  HC1.  The  following  were 
the  results  obtained: 

EXPERIMENT  v. 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Original  1.03  86.2 

Tailings  .39  5.6 

Corresponding  to  about:     93-5%  silver  extraction 

62.1%  gold  extraction. 

24 


It  becomes  evident  from  these  results  that  the  divergence 
between  the  results  of  the  two  preceding  tests  must  in  reality 
have  been  chiefly  due  to  the  difference  in  the  length  of  time 
of  treatments  rather  than  to  the  influence  of  the  increase  in 
the  percentage  of  iron  present  as  chloride  in  the  testing 
solution. 

In  order  to  further  test  this  method  of  extraction  of  precious 
metals,  another  ore  was  now  taken  up  and  experimented  with. 

EXPERIMENTS  WITH  ORE  NO.  2 

This  ore,  also  obtained  from  Tonopah,  was  a  reddish,  oxi- 
dized ore,  containing  much  ferric  oxide,  some  silver  sulphides, 
and  gold,  but  very  little  galena  or  lime.  It  was  crushed  to 
So-mesh  before  the  following  tests  were  made  with  it. 

Experiment  VI. — This  experiment  was  carried  out  under 
conditions  similar  to  those  of  Experiment  i,  only  500  grams 
of  ore  being  taken  instead  of  I  kilo. 

Although  in  this  ore  there  was  practically  no  metallic  iron 
to  depend  upon  to  furnish  the  ferric  salt  required  for  the 
attack  of  the  sulphides,  there  was,  however,  a  sufficiency  of 
Fe2O3,  occurring  as  such  in  the  ore,  to  serve  for  that  purpose 
by  going  into  solution  when  the  mixture  of  H2SO4  and  NaCl 
was  added.  The  iron  thus  brought  into  solution  amounted  to 
about  .4%  by  weight  of  the  solution. 

RESULTS    OF    EXPERIMENT   VI 

Au.  Ag. 

Sample  Oz.  per  ton. %  Oz.  per  ton. 

Original  .55  49.45 

Tailings  (hot  treatment)  .18  .5.5 

These  correspond  to  an  extraction  of  about :     89%      silver 

and     67.3%  gold. 

Experiment  VII. — In  order  to  determine  to  what  extent 
these  results  could  be  affected  by  adopting  the  conditions  of 
Experiment  IV,  namely,  by  a  cold,  three-day  treatment  in 
presence  of  chlorides  alone,  a  second  test  was  made  under 
the  latter  conditions,  with  the  following  results : . 

25 


EXPERIMENT  VII 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Original  .55  4945 

Tailings  (cold  treatment)  .075  7.3 

Corresponding  to  an  extraction  of:     85.7%   silver 

86.4%  gold. 

Although  no  further  experiments  were  carried  out  with  this 
ore  to  study  the  effect  of  a  finer  crushing  than  8o-mesh,  or 
of  a  more  prolonged  treatment,  it  is  safe  to  say,  however,  that 
had  these  measures  been  resorted  to,  the  above  results 
would  probably  have  been  much  improved  upon,  especially 
in  view  of  the  fact  that  it  was  noticed,  after  the  above  tests 
were  made,  that  the  ore  experimented  with  was  not  quite  of 
8o-mesh  crushing,  but  that  fully  15%  would  not  pass  through 
such  a  sieve. 

It  may  have  been  noticed,  from  the  descriptions  of  the  above 
ores,  that  they  were  both  remarkably  free  from  any  large 
amounts  of  the  sulphides  of  "base"  metals,  such  as  antimony, 
lead,  etc.  The  question  may  therefore  arise  here  as  to  whether 
the  method  of  ore  treatment  under  consideration  would  at 
all  successfully  apply  to  ores  containing  much  base  sulphides, 
and,  if  so,  to  what  extent  and  under  what  conditions. 

To  answer  this  question,  the  following  experiments,  under- 
taken with  the  above  point  in  view,  may  be  cited. 

EXPERIMENTS   WITH    ORE   NO.    3 

This  ore  was  obtained  from  Idaho.  It  was  a  dark  gray, 
almost  black,  mass,  chiefly  made  up  of  sulphides  of  antimony 
and  lead,  with  much  silver,  but  very  little  gold.  The  gangue 
was  silicious,  with  practically  no  lime  or  iron  oxide.  Besides 
the  above  sulphides,  there  was  a  small  amount  of  chalcopyrite 
and  pyrite  seen  scattered  through  the  mass.  The  ore  was 
coarsely  crushed  when  received.  An  assay  gave  the  following 
results : 

Original  ore:  161.8  ounces  silver  and  .12  ounces  gold  per  ton. 

Experiment  VIII. — Although  difficulties  in  treating  this  ore 
in  the  cold  might  have  been  expected  a  priori,  still  it  was 

26 


thought  of  interest  to  carry  out  some  preliminary  experiments 
with  it  under  conditions  as  nearly  economic  as  possible,  name- 
ly, in  the  cold  and  with  ordinary  crushing. 

First  Treatment. — The  ore  was  crushed  only  to  4O-mesh 
to  start  with,  and,  knowing  it  to  contain  not  much  iron  to 
depend  upon  to  supply  the  necessary  amount  of  oxidant  by 
going  into  solution  during  the  treatment,  the  mass  was  treated 
with  a  stock  solution  containing  2%  FeCl:l,  and  H,SO4  and 
NaCl  to  the  extent  of  3  and  20%  respectively.  The  weight 
of  ore  taken  being  300  grams,  the  volume  of  solution  used 
was  therefore  300  c.  c.,  that  is,  in  the  same  relative  proportion 
as  in  all  the  previous  experiments.  The  resulting  mixture 
of  ore  and  solution  was  kept  in  an  open  vessel  for  three  days 
at  room  temperature,  but  without  introducing  any  free  chlorine 
into  it. 

Twice  during  this  period,  however,  part  of  the  solution 
was  decanted  off  and  replaced  with  an  equal  volume  of  the 
fresh  stock  solution  so  as  to  facilitate  the  reactions  by  keeping 
a  good  supply  of  ferric  iron  on  hand  and,  at  the  same  time, 
by  removing  the  reaction  products,  especially  the  ferrous  iron, 
which  would  form  and  accumulate  in  the  solution  to  a  great 
extent. 

After  this  period  the  mass  was  thrown  on  a  filter  paper 
in  a  funnel,  leached,  washed  first  with  strong  salt  solution, 
then  with  pure  water,  and  finally  carefully  dried  and  assayed. 
The  following  were  the  results  thus  obtained: 

~  (100.1     oz.  silver  per  ton 

lailingfs,  ore  No.  3,  first  treatment   •( 

I        .06  oz.  gold     per  ton 

If  we  compare  these  results  with  those  of  the  assay  of  the 
original  ore,  we  see  that  this  corresponds  to  an  extraction  of 
only  about  38.2%  of  the  silver  and  50%  of  the  gold  present. 

Second  Treatment. — To  see  to  what  extent  a  prolongation 
of  treatment  could  improve  matters,  the  above  tailings  were 
treated  with  a  fresh  lot  of  the  stock  solution  and  allowed  to 
remain  in  contact  with  this  latter  for  three  days  more,  part  of 
the  solution  being  again  decanted  off  and  renewed  twice  during 
this  period.  The  results  obtained  were : 

(  82.5  oz.  silver  per  ton 
Above  tailings,  after  second  treatment  {  ^  "         r      . , 

j  Traces  of  gold  per  ton 

27 


Which  corresponds  to  an  extraction  of  about  49%  of  the 
silver 'present  and  practically  all  the  gold. 

It  is  to  be  noted  here  that  while  thus  far  the  extraction 
of  the  silver  cannot  be  called  satisfactory,  yet  that  of  the  gold 
is  strikingly  good.  To  what  was  the  latter  fact  due? 

Although  it  is  not  known  what  was  the  exact  state  of 
combination  of  the  gold  originally  present  in  the  ore,  it  is  \\-\ 
unlikely,  however,  that  this  metal  was  found  there  in  some 
readily  soluble  form,  such  that  it  was  gradually  removed  dur- 
ing the  successive  renewals  of  the  solution  by  decantation  or 
filtration.  The  fact  that  the  ferrous  iron — which  was  always 
present  in  the  solutions  during  the  treatments,  in  larger  or 
smaller  amounts — did  not  hinder  such  a  removal  of  the  gold 
must  have  been  due  first  to  the  extreme  smallness  of  the 
amount  of  this  metal  present  in  the  ore,  and  then  to  the  fact 
that  the  precipitation  of  gold  by  means  of  ferrous  salts  •« 
not  quantitative  when  other  oxidizing  agents,  such  as  fen  ir 
salts  or  atmospheric  oxygen,  are  also  at  hand. 

Third  and  Fourth  Treatments. — In  order  to  discover  whether 
finer  crushing  would  help  the  extraction  of  the  remaining 
silver  materially,  the  above  (second)  tailings  were  now 
crushed  down  to  a  fineness  of  70  meshes  and  then  subjected 
to  a  third  treatment  with  the  same  stock  solution  for  thr  *e 
more  days. 

However,  as  even  the  tailings  thus  resulting  assayed  63 
ounces  of  silver  per  ton  (corresponding  to  a  total  extraction 
of  61%),  a  fourth  treatment  of  the  same  tailings,  covering 
three  more  days,  was  next  undertaken,  this  time  crushing  the 
ore  down  to  100  meshes  and  increasing  the  percentage  of 
ferric  chloride  in  the  solution  to  5%.  This  last  treatment, 
however,  did  not  materially  alter  the  preceding  results,  as  the 
tailings  now  indicated  a  total  extraction  of  only  63%  of  the 
silver,  originally  in  the  ore,  during  the  four  treatments,  cover- 
ing a  total  period  of  twelve  days. 

This  naturally  rendered  it  necessary  to  use  the  agency  of 
heat  so  as  to  accelerate  the  reactions,  and  thereby  determine 
whether  the  fault  lay  in  the  process  itself  or  was  due  to  the 
character  of  the  ore. 

Before  doing  .this,  however,  it  was  thought  interesting  to 
decide  first  whether  it  was  the  length  of  time  of  treatment 


that  governed  the  degree  of  extraction  of  the  silver  in  the 
above  experiment,  or  whether  the  latter  depended  mainly  upon 
'the  fineness  of  division  of  the  ore. 

Experiment  IX. — To  answer  this  question,  another  experi- 
ment was  therefore  undertaken,  this  time  with  a  fresh  lot  of 
ore,  crushed  to  loo-mesh  to  start  with,  and  with  a  solution 
containing  Fed.,  to  the  extent  of  5%,  as  in  the  preceding 
test.  The  results  obtained  after  a  four  days'  treatment  in 
the  cold,  with  a  daily  decantation  and  partial  renewal  of  the 
solution,  showed  almost  exactly  the  percentage  of  extraction 
arrived  at  at  the  end  of  the  twelve  days  in  the  above-repeated 
tests  with  coarser  crushing. 

This  proved  definitely  that  the  ore  was  of  an  exceptionally 
refractory  character,  so  far  as  the  silver  contents  were  con- 
cerned. 

The  necessity  of  "forcing"  the  decomposition  of  the  sul- 
phides in  this  ore  by  carrying  the  treatment  in  the  warm, 
especially  if  finer  crushing  was  to  be  avoided,  became  the  more 
apparent  at  this  juncture,  as  it  was  noticed  at  the  end  of  the 
fourth  day,  even  in  the  preceding  test,  that  the  further  action 
of  the  solution  upon  the  ore  became  extremely  weak. 

Experiment  X. — A  fresh  3OO-gram  lot  of  the  4O-mesh  ore 
was  now  taken  and  a  series  of  tests  made  with  it,  by  boiling 
the  mass  in  a  flask  in  presence  of  a  solution  of  3%  H.,SO4, 
20%  NaCl,  and  5%  Fed,,  for  the  total  period  of  22  hours. 
The  treatments  lasted  from  three  to  six  hours  each,  the  solu- 
tions being,  of  course,  renewed  for  each  new  test.  The  fol- 
lowing is  a  tabular  statement  of  the  data  thus  obtained : 

EXPERIMENT  X.       HOT  TREATMENT  OE  ORE  NO.   3 

Ag.  An.           Per  cent 
oz.  per  ton.  oz.  per  ton.  extr.  silver 

Original  ore                                                         161.8  .12 

Tailings,   ist  treatment,  lasting  5  hrs.*         55.2  65.9 

Tailings,  2nd  treatment,  lasting  3  hrs.        36.1  777 

Tailings,  3rd  treatment,   lasting  3  hrs.         30.0  81.5 

Tailings,  4th  treatment,   lasting  6  hrs.         12.2  92.5 

Tailings,  5th  treatment,  lasting  5  hrs.             9.0  94.5 

At  the  end  of  these  22  hours  of  treatment,  the  solutions 
collected  were  analyzed  (approximately)  and  found  to  have 


*The  solution  was  once  decanted  and  renewed  at  the  end  of  the 
first  2  hours. 

29 


extracted  antimony  to  the  extent  of  3%  of  the  weight  of  the 
ore,  and  as  much  as  2%  of  lead,  besides  some  copper,  and 
silver  as  shown  above.  • 

From  these  results  it  appears  clearly  that  "Chloridation" 
will  succeed  even  with  ores  as  refractory  and  "base"  as  that 
under  consideration,  provided  the  agency  of  heat  is  introduced 
to  accelerate  the  decomposition  of  the  sulphides.* 

It  is  now  of  interest  to  consider  what  results  may  be  ex- 
pected of  this  method  of  ore  extraction  as  applied  to  very  poor 
ores  or  tailings. 

EXPERIMENTS  WITH  ORE  NO.  4 

This  ore  was  a  reddish,  oxidized  mass  containing  much 
limestone,  ferric  oxide,  copper  carbonate  to  the  extent  of 
.5%,  some  silver,  and  some  gold. 

Experiment  XL — As  the  ore  contained  much  CaCO3  and 
Fe2O3,  it  was  thought  to  utilize  these  by  using  a  rather  strong 
solution  of  HC1  alone  for  the  lixiviating  agent,  and  thereby 
omitting  the  introduction  of  the  chlorides  of  sodium  and  iron. 

Two  hundred  grams  of  the  ore  were  therefore  taken  and 
treated  with  the  above  acid,  so  that  when  all  the  carbonates 
had  been  completely  decomposed,  there  was  an  excess  of  10% 
HC1  left  free  in  the  total  volume  of  200  c.cs.  of  solution. 

Although  the  ore  contained  a  small  percentage  of  MnO2, 
which,  reacting  upon  the  HC1  present,  gave  some  free  chlorine, 
nevertheless,  in  order  to  insure  the  attack  of  the  gold,  the  mass 
was  gently  chlorinated  and  kept  at  room  temperature  for  three 
days,  with  occasional  shaking.  The  following  are  the  results 
thus  obtained : 

EXPERIMENT    XI 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Original  .09  4.2 

Tailings  .02  1.4 

Which  correpsonds  to  about:     66.6%  silver  extraction 

and     77-8%  gold    extraction. 

In  order  to  ascertain  whether  or  not  these  low  results  were 


*The  possibility  of  giving  a  preliminary  roasting  to  such  ores   ii 
lieu  of  heating  the  solution  has  not  yet  been  considered. 


referable  merely  to  the  conditions  of  experimentation,  several 
other  experiments  were  carried  out  under  varying  conditions 
as  to  the  composition  of  the  solution,  using  H2SO4  and  NaCl, 
etc.,  but  it  was  found  that  in  nowise  could  the  above  results 
be  improved.  Of  course,  the  possibility  of  using  heat  in  con- 
nection with  the  treatment  of  such  low-grade  ores  in  practice 
being  out  of  the  question,  no  attempt  was  made  to  test  the 
ore  with  hot  solutions.  It  is  well  to  say  here,  however,  that 
just  as  it  is  likely  that  such  hot  treatments  would  give  better 
extractions  with  the  above  ore,  as  with  the  others,  by  disinte- 
grating the  coarser  grains  in  it — to  whose  presence  these  low 
results  are  probably  due — just  so  it  is  probable  that,  had  these 
coarser  particles  been  ground  finer,  the  extraction  would  have 
been  improved,  even  with  ordinary  cold  treatments.  Never- 
theless, it  seems  doubtful  if  tailings  of  the  type  of  the  above, 
namely,  those  rich  in  Fe2O3,  can  ever  be  totally  deprived  of  the 
precious  metals,  owing  to  the  fact  that  such  ores  are  likely 
to  retain  by  occlusion,  as  is  well  known,  considerable  amounts 
of  chlorides,  those  of  silver  and  gold  among  others,  in  a  man- 
ner difficult,  if  not  impossible,  to  obviate. 

Besides  the  above  ores,  others  were  experimented  with, 
some  mere  tailings,  others  fairly  rich  in  silver  and  gold.  With- 
out going  into  a  detailed  account  of  these  various  experiments, 
the  following  conclusions,  based  upon  the  experience  thus  far 
gained  by  the  writer,  may  be  safely  drawn : 

1.  The  Chloridation  method,  as  above  described,  may  well 
be  depended  upon  to  extract  the  precious  metals   from  their 
ordinary  ores  of  good  grade  in  a  very  satisfactory  manner, 
especially  as  regards  the  extraction  of  the  silver  values. 

2.  The  thoroughness  of  extraction  of  the  gold  values  de- 
pends entirely  upon  the  degree  of  oxidation  of  the  reacting 
solution  by  means  of  chlorine. 

3.  In  many  cases,  with  silver  ores  carrying  gold,  a  suffi- 
cient percentage  of  this  latter  metal  is  also  extracted,  besides 
the  silver,  to  pay  a  large  share,  if  not  the  whole,  of  the  ex- 
penses of  treatment,  and  this  with  no  more  care  than  is  to  be 
given  for  the  extraction  of  silver  alone. 

4.  With  very  low-grade  ores,  or  tailings  rich  in  oxide  of 
iron,  the  highest  extractions  can  only  be  had  economically  by 
fine  crushing. 


5.  With  very  base  ores  of  good  grade  the  process  is  too 
slow  to  be  paying  if  carried  out  in  the  cold.     In  such  cases, 
unlike  with  ordinary  ores,  the  agency  of  heat,  must  be  resorted 
to,  either  by  way  of  warming  the  solutions  or  by  giving  a 
preliminary  roasting  to  the  ores. 

6.  In  practice  the  solution  used  in  any  case  may  be  either 
salt   and   sulphuric   acid,   or   salt    (or   calcium   chloride)    and 
hydrochloric   acid,   or   hydrochloric   acid   alone,   or  any   other 
suitable   combination,   according   to   local   economic    facilities. 
As  to  the  supply  of  iron  salts  needed,  it  is  probable  that  it 
will  seldom  be  necessary  to  introduce. such  salts  into  the  main 
solution  from  without,  since  ordinarily  ores  will  be  found  to 
contain  enough  of  the  compound  of  this  metal  in  a  soluble 
form  to  furnish  all  the  oxidizing  agents  necessary  to  enable 
the  solution  to  do  its  work  properly,  once  these  salts  of  iron 
are  brought  to  the  ferric  condition. 


ELECTROLYSIS 

There  remains  now  to  consider  the  electrolysis  of  the  solu- 
tions, with  a  view  to  recovering  the  precious  metals  dissolved 
thereby,  oxidizing  the  ferrous  salts,  and  therefore  regenerating 
the  ferric  salts  and  the  solution  as  a  whole. 

So  far  as  the  recovery  of  the  metals  is  concerned,  it  seems 
to  be  safe  to  say  that,  in  view  of  the  many  vain  attempts  made 
by  the  writer  to  obtain  good  adherent  deposits  on  the  cathodes, 
it  is  useless  to  trouble  one's  self  by  trying  to  get  any  such 
deposits  in  large-scale  practice. 

Without  going  into  any  unnecessary  accounts  as  to  the 
details  of  these  various  attempts,  it  is  sufficient  to  say  here  that 
if  the  solution  be  kept  in  motion  during  the  electrolysis,  which 
is  a  necessity  for  continuous  work,  it  is  found  that  a  current 
density  of  .1  ampere  per  square  decimeter,  with  a  voltage  of 
1.5  to  2  volts,  and  with  platinum  electrodes,  will  not  deposit 
the  values  on  the  cathode  at  all;  whereas  this  same  current 
will  succeed  in  precipitating  all  the  values  if  the  solution 
be  kept  quiescent.  The  reason  for  this  peculiarity  lies, 
undoubtedly,  in  that  the  deposit  being  slimy  or  spongy 
and  easily  rubbed  off  the  surfaces  of  the  cathodes  by  the 

32 


solution  in  motion,  this  latter  thus  has  a  stronger  ten- 
dency to  re-dissolve  the  values  than  has  a  current  of  the  given 
density  to  precipitate  them.  By  increasing  the  current  den- 
sity, however,  the  deposition  is  carried  to  completion,  although 
as  slimy  as  ever. 

If,  however,  a  very  small  percentage  (the  fraction  of  a  per 
cent)  of  glue  or  any  mucilaginous  substance  is  added  to  the 
solution,  the  character  of  the  deposit  obtained  is  wholly 
changed.  Instead  of  the  loose  and  black  precipitate  on  the 
cathode,  one  now  gets,  as  a  rule,  especially  if  the  solution  be 
warmed,  a  good,  firm,  and  whitish  gray  deposit,  which  takes 
a  high  polish  when  rubbed  with  the  handle  or  back  of  a  knife. 
Moreover,  it  now  matters  no  longer  whether  the  solution  is 
left  quiescent  or  stirred  vigorously  or  even  kept  boiling.  The 
deposit  is  always  good,  provided  the  current  density  is  proper. 
Thus,  by  reason  of  the  very  fact  that  this  deposit  sticks  firmly 
to  the  cathode,  it  would  seem  as  if  with  a  small  "trick"  like 
this  one  might  get  good  and  firm  deposits  and  thus  stop  the 
values,  once  deposited  on  the  cathodes,  from  falling  off  into 
the  solution  and  there  being  subject  to  redissolution. 

Unfortunately,  however,  sometimes,  in  spite  of  the  best  in- 
tentions and  care  on  the  part  of  the  operator,  the  solution 
refuses  to  give  good  deposits,  even  with  the  addition  of  glue. 
And  then  the  deposit  is  again  slimy  and  black.  For  these 
and  other  reasons  liable  to  cause  possible  hitches  in  the  work 
of  electrolysis,  in  practice  it  might  be  better  to  have  recourse 
to  some  mechanical  device  whereby  the  slimy  metals  on  the 
cathodes  may  be  continuously  removed  before  they  have  any 
chance  to  fall  off  into  the  solution. 

A  handy  method   in  point   is  one  described  by  Tomassi.* 

Briefly  described,  it  lies  in  that  the  cathodes  are  made 
of  circular  plates,  only  half  of  each  of  which  dips  into 
the  solution.  They  are,  further,  so  arranged  that  they  can 
rotate  at  a  slow  rate,  say  once  or  twice  a  minute.  In  conse- 
quence of  this  rotation  there  is  always  some  new  surface  being 
covered  with  slimy  metal  deposit,  while  other  portions  emerge 
from  the  solution  already  so  charged.  In  its  motion  the  ex- 
ternal part  of  a  plate  comes  to  rub  itself  against  some  rubber 


*Comptcs  Rcndus,  Academic  des  Sciences  de  Paris,  vol.  122,  p.  1476. 

33 


pads  provided  with  long  grooves  leading  to  a  suitable  recep- 
tacle near  by.  Along  these  grooves,  serving  as  channels,  the 
slimes,  loosened  by  the  friction  between  rubber  pads  and 
cathode  plates,  are  drifted  by  means  of  small  jets  of  a  dilute 
solution  of  salt,  and  thus  carried  into  the  receptacle  where  the 
values  are  thus  collected. 

As  for  the  materials  of  which  the  electrodes  may  best  be 
made  in  connection  with  this  work,  it  is  obvious  that  carbon 
or  graphite  would  be  the  most  suitable  as  regards  durability, 
desirability,  and  cheapness,  especially  as  no  other  cheap  metal 
could  stand  unattacked  in  such  solutions  of  ferric  salts. 

As  regards  the  oxidation  of  the  ferrous  salts  in  solution 
or  the  regeneration  of  the  ferric  salts,  the  writer  is  in  position 
to  state  here  that,  in  order  to  bring  this  about  by  means  of 
the  electric  current,  there  is  absolutely  no  necessity  of  using 
any  diaphragm  cells  of  any  sort.  If  the  current  density  be 
properly  adjusted  and  the  metals  deposited  or  satisfactorily 
removed  from  solution,  whether  by  using  glue  or  by  means 
of  the  above,  or  any  other,  mechanical  contrivance,  the  oxida- 
tion of  the  ferrous  salts  accumulated  in  the  solution  during  the 
latter 's  passage  through  the  ore  mass  can  go  on  concurrently 
with  the  electro-deposition  of  the  metals,  so  that  ultimately 
there  are  not  only  no  ferrous  salts  left  unoxidized,  but  what 
is  more,  the  solution  will  be  found  to  be  charged  with  free 
chlorine.  It  is  important  to  call  special  attention  to  this  fact, 
inasmuch  as  certain  authorities  and  all  the  inventors  of 
"patent''  methods,  who  have  so  much  as  specified  the  use  of 
the  electric  current  for  the  regeneration  of  the  solution,  have 
persistently  thought  it  to  be  impossible  not  only  to  thus  re- 
oxidize  the  iron  salts  in  solution,  but  have  actually  feared  that, 
instead,  the  higher  oxides  of  this  and  other  metals  dissolved 
might  be  reduced  to  lower  ones;  and  they  have,  therefore, 
advocated  the  use  of  some  form  or  other  of  porous  diaphragms 
to  separate  the  cathodes  from  the  anodes  in  the  electrolytic 
cells. 

When  it  is  remembered  that  diaphragms  are  not  the  best 
things  that  could  be  wished  for  or  even  allowed  in  large-scale 
practice,  and  in  solutions  of  the  character  here  under  conside  i- 
tion,  the  importance  of  the  foregoing  observation  can  scarcely 
be  lost  sight  of. 

34 


In  actual  practice,  what  should  be  the  mechanical  and  othr; 
arrangements  to  be  adopted  in  treating  ores  on  a  serious  scale 
by  the  electro-chemical  "Chloridation"  method  here  advanced? 

It  would  certainly  be  idle  to  enter  here  into  any  details  of 
the  possible  contrivances  which  might  be  devised  and  put  into 
use  under  such  circumstances.  Large-scale  practice  alone  \\  i!l 
decide  questions  of  that  character. 

Suffice  it  to  say  here,  however,  that  in  putting  the  method 
into  practical  use,  the  governing  principles  of  operation  w:!! 
be  the  following: 

The  ores,  suitably  crushed,  are  to  be  kept  in  wooden  vats 
and  treated  with  the  stock  solution  best  by  simple  leaching. 

The  teachings  thus  collected  are  made  to  pass  continuously 
through  simple  electrolytic  vats  where,  on  the  one  hand,  the 
dissolved  metals  are  recoverd  and,  on  the  other,  the  solution 
is  reoxidized,  or  "regenerated,"  under  the  influence  of  the 
electric  current.  The  number  of  cells  and  electrodes,  the  cur- 
rent strength,  and  the  flow  of  the  solution  so  arranged  that, 
by  the  time  the  solution  leaves  the  last  cell,  it  is  satisfactorily 
impoverished  of  ;i:s  precipitable  metals  and  completely  oxidized, 
chlorinated,  in  other  words,  fully  regenerated;  which  done, 
it  is  ready  to  be  led  over  into  the  ore  vats  again,  allowed  to 
leach  through  the  ore  masses,  then  passed  through  the  electro- 
lytic cells,  and  finally  returned  to  the  ore  vats  over  again,  and 
so  on,  cycle  after  cycle. 

When  a  given  batch  of  ore  is  sufficiently  impoverished  of 
its  metals,  it  is  treated  with  pure  water  so  as  to  displace  and 
recover  the  stock  solution  impregnating  the  exhausted  ore. 

To  what  extent  this  latter  operation  may  be  economically 
carried  out,  and  what  other  modifications,  in  the  line  of  sec- 
ondary treatments,  may  be  introduced  into  the  process  as  a 
whole,  will  be  alluded  to  in  the  second  part  of  this  paper. 


35 


II.   PERSULPHATATION 


HISTORICAL 

Unlike  solutions  of  cupric  and  ferric  salts,  and  in  spite  of 
the  fact  that  they  have  come  latterly  to  be  regular  articles  of 
commerce,  persulphates  do  not  seem  to  have  as  yet  been  used, 
or  even  suggested,  as  direct  agents  for  extracting  metals  from 
their  ores.  There  seems  to  be  one  allusion  made  in  the  litera- 
ture on  the  subject*  to  the  effect  that  persulphates  could  be 
used  in  conjunction  with  cyaniding  as  a  substitute  for  oxida- 
tion by  aeration,  which  is  well  known  to  be  essential  for  the 
hastening  of  the  solvent  action  of  the  cyanides  upon  gold. 
Although  it  appears  even  that  a  patent  has  been  issued  to  a 
German  company§  to  cover  this  particular  use  of  persulphates 
along  with  cyaniding,  yet  nothing  seems  to  be  known,  in  prac- 
tical mining  circles,  of  this  method  of  ore  treatment  having 
ever  been  put  into  actual,  large-scale  operations  anywhere. 

Considering  the  fact,  however,  that  cyanides  are  decomposed 
by  persulphates, t  it  is  not  at  all  unlikely  that  this  "patent" 
suggestion,  like  so  many  others,  has  never- as  yet  left  the  con- 
fines of  patent  specifications. 


THEORETICAL 

In  the  method  of  metal  extraction  here  to  be  described,  the 
chief  point  of  interest  lies  in  that  both  the  oxidation  of  the 
sulphides  of  silver  and  of  gold  is  brought  about  under  the 
influence  of  one  and  the  same  compound,  namely,  the  persul- 
phate used,  under  proper  conditions,  but  without  any  necessary 
dependence  upon  the  generation  of  chlorine  by  electrolysis. 


*Elbs,  Ztsch.  Angew.  Chemic,  '97,  p.  195. 
^Journal  Am.  Chem.  Soc.,  '97,  vol.  19,  p.  900. 
JN.  Tarugi,  Centrlbl.,  '03,  p.  616. 

36 


In  bringing  this  double  effect  about,  advantage  is  taken  of 
the  fact  that  persulphates  decompose  chlorides,  setting  chlorine 
free,  ready  to  attack  gold,  especially  in  presence  of  free  acids, 
and  that,  on  the  other  hand,  they  attack  sulphides,  readily 
converting  them  into  sulphates,  while  they  themselves  break 
down  to  simple  sulphates. 

The  reaction  taking  place  in  the  first  case  seems  to  be  of  the 
type : 

2NaCl+K2S2O8+H2SO4  =  Na2SO4+K2SO4+H2SO4+Cl2 

the  decomposition  of  the  chloride  being  referable  to  the  forma- 
tion, first,  of  free  persulphuric  acid,  thus : 

KaS3O8+HaSO4  =  K2SO4+H2S,OS 

which  then  reacts  upon  the  chlorides  in  a  manner  now  well 
known : 

2NaCl+H,S,OH  =  Na2SO4+H2SO4+Cl2 

While  with  the  ordinary  concentrations  of  acids  and' persul- 
phates the  attack  of  chlorides  can  be  called  all  but  rapid  at 
best — a  fact  which  is  rather  fortunate  from  the  practical 
standpoint,  seeing  that  the  nature  of  the  work  to  be  done  by 
the  chlorine  thus  liberated,  namely,  the  solution  of  gold,  re- 
quires only  a  slow  but  continued  oxidizing  effect — that  of  the 
sulphides,  on  the  other  hand,  is  much  more  readily  accom- 
plished. In  fact,  this  decomposition  goes  on  readily,  even  in 
neutral  or  slightly  alkaline  solutions,  though  it  is  certainly 
more  vigorous  in  the  presence  of  acids.  The  reactions  here 
in  question  seem  to  take  place  as  follows : 

Ag2S+K2S2O8  =  Ag2SO4+K2SO4+S 

though  much  of  the  sulphur  forming  the  metallic  sulphides 
is  oxidized  completely  over  to  sulphuric  acid. 

These  being  the  mechanisms  and  character  of  the  re- 
actions immediately  involved  in  the  use  of  solutions  of 
persulphates  for  the  dissolution  and  extraction  of  the  pre- 
cious metals,  it  is  evident  that,  as  with  the  chloridation 
method,  so  also  here  it  is  essential  for  the  successful  work- 
ing of  the  present  method  in  actual  operations  to  have  these 
solutions  charged  with  a  soluble  chloride.  The  introduc- 
tion of  such  chlorides  is,  indeed,  important  in  this  case,  not 

37 


merely  because  it  is  necessary  to  have  some  chloride  pres- 
ent in  the  solution  so  as  to  react  with  the  persulphates 
and  liberate  the  chlorine  necessary  to  dissolve  the  gold  in 
the  ore,  but  also  because,  AgCl  being  no  more  soluble  in 
simple  persulphate  solutions  than  in  ordinary  solutions  of 
iron  salts,  there  is  here  the  same  danger  of  much  silver 
being  lost  in  consequence  of  the  unavoidable  presence  of 
chlorine  compounds  occurring  as  impurities  in  ores,  unless 
some  appropriate  solvent  of  AgCl  is  used  here  as  with  the 
chloridation  method. 

But  there  is  even  a  more  grave  reason  for  which  such 
a  solvent  of  silver  should  be  used  in  this  connection,  and 
that  is  that  silver  salts  form  an  insoluble  precipitate  of 
peroxide  of  silver  (AgD),  in  presence  of  persulphates,  which 
is  only  decomposed  and  dissolved  satisfactorily  by  strong 
chloride  solutions. 

In  endeavoring  to  select  an  appropriate  solvent  for  these 
insoluble  silver  compounds,  it  was  at  first  thought  that, 
unlike  with  the  previous  method,  ammonia  would  be  a  de- 
sirable substance  to  use  in  conjunction  with  the  present 
method  of  ore  extraction,  inasmuch  as  it  would  act  as  a 
very  efficient  solvent  of  any  silver  chloride  which  might 
form  in  the  course  of  the  treatment,  while,  at  the  same  time, 
the  action  of  persulphates  upon  the  sulphides  would  not 
thereby  be  impaired  in  any  way.  This  belief  had,  however, 
soon  to  be  abandoned.  It  was,  indeed,  observed  that,  far 
from  being  a  suitable  substance  for  the  purpose,  ammonia 
was  decidedly  objectionable,  inasmuch  as  it  was  found  to 
be  rapidly  decomposed  into  free  nitrogen  and  water  when- 
ever any  appreciable  amount  of  silver  and  a  persulphate 
were  found  in  solution  with  it.  This  oxidation  of  ammonia 
under  these  circumstances  is  due,  doubtlessly,  to  a  catalytic 
reaction  depending  upon  the  formation,  primarily,  of  the 
peroxide  AgO,  which  then  attacks,  more  or  less  violently, 
any  free  ammonia  which  may  be  present  in  the  solution, 
and  causes  its  complete  decomposition  in  a  well-known 
manner.* 

The  reaction  is,  indeed,  so  characteristic  that  the  writer  has 


*Dammer's  Handbuch  Anorganischen  Chemie. 

38 


found  it  to  be  a  very  handy  way  for  detecting  the  presence 
of  silver  salts  in  chloride  solutions,  which  is  the  more  useful 
as  it  is  oftentimes  impossible  to  detect  silver  in  such  solutions, 
whether  by  any  other  simple  precipitation  method  or  by  a  mere 
dilution  of  the  solution  with  the  hope  of  thus  diminishing 
the  solvent  power  of  the  liquid  on  AgCl  and  thereby  causing 
a  turbidity  of  this  compound  to  appear.  In  carrying  out  this 
test,  a  strong  solution  of  a  persulphate,  preferably  the  am- 
monium salt,  is  added  to  the  solution  to  be  tested,  and  then 
this  latter  is  made  strongly  alkaline  with  NH4OH,  when,  if 
silver  be  present,  a  more  or  less  rapid  evolution  of  nitrogen 
gas  will  be  observed  to  take  place.* 

The  use  of  ammonia  as  a  solvent  of  insoluble  silver  salts 
being  thus  out  of  the  question,  and  none  of  the  other  non- 
chloride  solvents  being  sufficiently  stable  in  presence  of  per- 
sulphates,  the  choice  of  concentrated  solutions  of  a  chloride 
thus  becomes  a  necessity. 

Now,  as  it  is  remembered  that  the  use  of  persulphates  as 
reagents  for  treating  ores  of  the  precious  metals  was  suggested, 
on  an  earlier  page  of  this  paper,  on  the  ground  that  they  were 
good  oxidants,  capable  of  being  regenerated  by  the  electric 
current;  and,  further,  as  it  is  realized  here  that,  though  a 
small  percentage  of  a  chloride  in  concentrated  solutions  of 
sulphates  is  very  advantageous  for  the  electrolytic  formation 
or  regeneration  of  persulphates,§  higher  percentages  of  chlo- 
rides are  quite  injurious  to  such  regeneration,  the  question 
may  arise  here  as  to  whether  the  means  above  stated  for  hold- 
ing silver  salts  in  solution  in  actual  practice  would  not  render 
the  process  as  a  whole  inoperative,  at  least  as  a  "cyclic"  meth- 
od. It  is,  indeed,  only  too  evident  that,  an  essential  condi- 
tion for  the  formation  of  persulphates  by  means  of  the  electric 
current  being  a  high  anodic  current  density,  or  better,  a  high 
rate  of  discharge,  per  unit  of  area  and  time,  of  SO4  ions  at  the 
anodes,  the  introduction  of  unduly  large  proportions  of  a 
non-sulphate  electrolyte,  such  as  NaCl  for  instance,  will  tend 
to  diminish  the  yield  of  the  current  in  persulphate  formation. 


*This  catalytic  reaction  has  also  been  studied  lately  and  similarly 
explained  by  Hugh  Marshall   (7.  Chem.  Soc.,  '01,  Abstr.  ii,  p.  156). 

$Elbs,  Ztschr.  fur  Elektrochemie,  '95,  p.  245. 

39 


How  such  a  serious  difficulty  may  be  overcome  in  practice 
will  be  discussed  further  on.  Suffice  it  to  say  here  only  that, 
were  there  even  no  way  of  surmounting  this  difficulty,  the  fact 
that  persulphates  are  becoming  more  and  more  readily  avail- 
able as  regular  chemicals  of  commerce,  so  that  they  could  be 
economically  secured  for  practical  usage,  this  fact  justifies  the 
study  here  of  their  suitability  for  the  extraction  of  the  precious 
metals  from  their  ores. 

As  for  the  question  which  may  arise  here  regarding  the 
advantages  such  a  method  may  have,  at  best,  over  the  chlori- 
dation  method  previously  described,  the  following  points  may 
be  cited  in  answer : 

1.  Theoretically  the  use  of  a  persulphate  should  be  prefer- 
able to  that  of  ferric  salts  alone,  because,  unlike  the  latter, 
whose   reaction   products,   namely,   the   ferrous   salts,   are   no- 
torious for  their  tendency  to  reprecepitate  the  precious  metals 
from   their  dissolved   state,   persulphates   break   down   merely 
to   sulphates,   that   is,   salts  having  no   such   injurious   effects 
upon  the  solutions  of  these  metals. 

2.  For  this  reason  principally,  simple  gold  ores  could  be 
treated  by  persulphatation  to  a  better  advantage  than  by  chlo- 
ridation,  or  perhaps  by  any  other  wet  method. 

3.  In  practice  one  may  have  to  treat  ores  that  are  too  rich 
in  limestone  to  be  economically  handled  by  the  chloridation 
process,  inasmuch  as  they  would  first  have  to  receive  a  pre- 
liminary acid  treatment  before  they  could  be  subjected  to  the 
action  of  ferric  salts,  lest  these  latter  should  be  precipitated 
out  and  the  process  rendered  more  or  less  inoperative.     Such 
ores  should  be  amenable  to  persulphatation  to  a  better  advan- 
tage,   seeing   that    persulphates    can    decompose    sulphides    of 
silver,  as  was  stated  above,  in  neutral  or  slightly  alkaline  media 
as  well  as  in  acid. 

On  the  other  hand,  it  is  only  fair  to  admit  here  that,  with 
very  "base"  ores,  persulphatation  cannot  be  expected  to  be 
nearly  as  useful  a  method  as  chloridation,  owing  to  the  fact 
that  the  satisfactory  oxidation  of  such  ores  by  wet  methods 
such  as  these,  requiring,  as  they  do,  the  heating  of  the  reacting 
solutions,  persulphates  would  be  rapidly  decomposed  under 
these  conditions  and  hence  be  useless  as  economic. agents  for 
treating  this  type  of  ores. 

40 


To  what  extent  the  above  premises  may  be  justified  will 
now  be  studied  in  a  quantitative  way. 

In  the  following  experiments  the  persulphate  worked  with 
has  been  the  potassium  salt  throughout.  This  choice  was 
made  for  two  chief  reasons.  In  the  first  place  because  sodium 
persulphate,  whose  use  would  suggest  itself  first,  is  unfor- 
tunately not  to  be  had  on  the  market  here,  nor  can  it  be  made 
readily  in  the  solid  condition.  In  the  second  place  because 
the  ammonium  salt,  though  it  is  of  all  the  persulphates  the 
one  most  readily  made  or  got,  is  not  adapted  for  experiments 
of  this  kind,  let  alone  for  large-scale  use,  owing  to  the  fact 
that  it  is  rather  unstable  and  easily  decomposed  into  free 
nitrogen  or  its  oxides  by  auto-oxidation,*  and  especially  under 
the  influence  of  the  chlorine,  which  would  tend  to  be  liberated, 
whether  during  the  treatment  of  the  ores  or  during  the  electro- 
deposition  of  the  metals. 

As  none  of  these  disadvantages  is  seriously  shared  with  by 
potassium  persulphate,  hence  its  selection  for  the  following 
study. 

EXPERIMENTAL 

EXPERIMENTS    WITH    NON-ALKALINE    ORES 

As  both  the  decomposition  of  sulphides  and  the  solution 
of  gold  can  be  best  brought  about  by  persulphates  when  some 
free  acid  is  present,  it  is  obvious  that  whenever  the  use  of 
acid  solutions  is  permissible  they  should  be  resorted  to  in 
practice.  In  fact,  there  is  a  second  advantage  gained  in  such 
a  practice,  namely,  that  under  these  conditions  there  are  also 
more  or  less  ferric  salts  introduced  into  the  solution  through 
the  agency  of  the  acidity  of  the  liquids  upon  the  compounds 
of  iron  which  may  be  in  the  ores,  and  hence  the  oxidizing 
power  of  the  solution  is  thereby  enhanced. 

The  following  experiments  were  carried  out  with  a  view  to 
studying  the  effect  of  persulphates  under  these  conditions: 

ORE    NO.    2 

This  ore  was  that  already  described  on  a  previous  page. 
As  it  represented  more  nearly  the  average  type  of  good  ores 

*Marshall,  /.  Chem.  Soc.    (London),  '01,  abstr.  ii,  p.   156. 

41 


ordinarily  met  with,  it  was  thought  best  to  study  it  first  in 
this  connection. 

In  order  to  form  an  idea  as  to  the  possibilities  of  the  method 
in  a  general  way  as  a  means  of  precious  metal  extraction,  a 
set  of  experiments  was  undertaken  in  a  preliminary  way. 

Experiment  XII. — The  solution  used  in  these  tests  was  made 
up  of  10%  H2SO4,  20%  NaCl,  and  2%  K,S2O8.  The  tests 
were  carried  out  on  a  lot  of  300  grams  of  ore,  with  the  usual 
corresponding  bulk  of  solution,  namely,  300  c.cs.  in  the  pres- 
ent case.  The  mass  was  put  into  a  bottle  and  left  standing  in 
the  cold  during  a  period  of  3  days,  with  frequent  shaking. 
At  the  end  of  each  24  hours,  however,  it  was  thrown  on  a 
filter,  leached,  washed,  and  dried,  exactly  as  with  the  tests 
previously  described  in  connection  with  the  chloridation  meth- 
od, and  finally  a  sample  was  taken  and  assayed.  This  done, 
the  remaining  tailings  were  treated  with  a  solution  of  the 
same  concentration  as  above  mentioned,  for  a  second  and  third 
time,  only  using  a  fresh  solution  each  time. 

The  following  were  the  results  obtained : 

EXPERIMENT  xii 

Au.  Ag.        Per  cent  Extr. 

Sample  oz.  per  ton.  oz.  per  ton.  Gold.  Silver. 

Original  .55  49.45 

Tailings   after    ist   day's   treatment         .092  7.5  83.3      84.9 

Tailings  after  a  2nd  day's  treatment     .032  4.5  94.2      90.9 

Tailings  after  a  3rd  day's  treatment     .031  3.9  94.2      92.1 

These  results  clearly  show  a  decided  improvement  over 
those  obtained  previously  by  chloridation  alone. 

As  it  is  stated  here  that  during  these  treatments  there  was 
a  strong  odor  of  chlorine  evolved  by  the  solution,  it  may  be 
lightly  thought  a  priori  that  the  thoroughness  of  the  extraction 
of  gold  is  due  to  the  presence  of  this  gas  in  solution.  As  it  is' 
remembered,  however,  that  this  same  gas  was  also  present  in 
the  solution  with  the  chloridation  tests  carried  out  with  the 
same  ore,  and  even  in  larger  amounts,  it  becomes  evident  that 
the  present  remarkably  high  degree  of  extraction  of  the  gold  is 
due  to  more  than  the  mere  presence  of  chlorine  in  solution.  It 
seems,  indeed,  that  the  chlorine  on  hand  in  the  solution  in 
the  present  experiments  being  nascent,  while  that  in  the  chlo- 
ridation tests  was  molecular,  the  favorable  difference  here 

42 


observed  should  be  referred  to  this  particular  fact  of  the  chlo- 
rine being  here  in  a  nascent  state.  If  so,  this  would  constitute 
another  advantage  of  persulphatation  over  chloridation  proper, 
or  perhaps  any  other  form  of  chlorination,  especially  as  a 
method  of  gold  extraction. 

Now,  since  the  percentage  of  free  acid  used  in  the  above 
preliminary  tests  was  somewhat  unduly  too  high  from  the 
practical  standpoint,  it  was  of  interest  to  study  the  effect 
of  varying  the  amount  of  the  free  acid  in  the  solution  on  the 
extraction  of  the  metals,  with  a  view  to  using  lower  percentages 
in  practice. 

Experiment  XIII.  —  A  new  set  of  tests  was  therefore  under- 
taken, using  two  solutions,  one  containing  3%,  the  other  5% 
H2SO4,  with  2%  of  K2S2OS  and  the  normal  amount  (20%) 
of  NaCl  in  each.  The  treatment,  carried  out  as  before  in  all 
other  respects,  lasted  only  24  hours,  and  gave  the  following 
results  for  the  tailings  assayed  : 


XTIT 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Tailings  (3%  H2SO4)  .3  5.75 

Tailings  (5%  H2SO4)  .22  5.1 

From  these  results  it  would  appear  that  the  chief  function 
of  the  acid  lies  in  that  it  improves  the  extraction  of  gold 
alone,  while  that  of  silver  is  left  practically  constant. 

Experiment  XIV.  —  In  order  to  decide  whether  this  rela- 
tionship was  merely  fortuitous  or  not,  another  set  of  experi- 
ments was  carried  out,  this  time  using  a  solution  of  only  i% 
K2S2O8,  but  extending  the  time  of  treatment  to  48  hours,  with 
a  daily  decantation  and  renewal  of  the  solutions,  the  acidity 
in  the  latter  being  again  3%  and  5%  respectively. 

EXPERIMENT    XIV 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Tailings  (3%  H2SO4)  .30  9.0 

Tailings  (5%  H2SO4)  .163  8.4 

Whence  it  is  clearly  seen  that  in  reality  the  chief  function 
of  the  acid  in  these  solutions  lies  in  that  it  facilitates  the  dis- 

43 


solution  or  extraction  of  the  gold  rather  than  that  of  the 
silver  values.  In  other  words,  the  alteration  of  the  concen- 
tration of  the  acidity  in  these  solutions  affects  the  extraction 
of  the  first  metal  rather  than  that  of  the  second.  And  the 
general  results  of  these  experiments  have  been  corroborated 
in  a  third  set  of  tests. 

This  point  having  been  thus  definitely  ascertained,  it  became 
next  important  to  see  what  effect  the  variation  of  the  concen- 
tration of  the  persulphate  would  have  upon  the  extraction 
of  these  precious  metals. 

Experiment  XV. — For  this  purpose  two  tests  were  made, 
using  two  solutions  of  i%  and  2%  K2S2O8  respectively,  while 
both  were  acidified  to  the  extent  of  3%  H2SO4.*  The  results, 
after  24  hours'  contact  of  the  ore  with  the  solutions,  were : 

EXPERIMENT  XV 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Tailings  (i%  K2S2O8)  .30  13.3 

Tailings  (2%  K2S208)  .30  5.7 

Strange  as  it  may  seem,  these  results  clearly  indicate  that 
the  variation  in  the  concentration  of  the  persulphate  in  solu- 
tion affects  especially  the  extraction  of  silver  alone.  In  other 
words,  the  function  of  the  persulphate  would  seem  to  be  espe- 
cially one  of  oxidation  of  the  silver  compounds. 

Experiment  XVI. — In  order  to  ascertain  that  this  was  not 
either  a  mere  fortuitous  coincidence,  a  new  test  was  carried 
out  using  a  solution  containing  3%  H2SO4  and  2%  K2S2O8, 
but  extending  the  time  of  treatment  to  48  hours.  This  was 
done  so  as  to  be  able  to  compare  it  with  the  results  of  a  pre- 
vious test  (given  in  table  preceding  the  last),  where  only 
i%  K2S2O8  was  used,  with  also  3%  H2SO4  and  a  48-hour 
treatment.  The  following  is  a  statement  of  the  results  of  these 
two  tests: 


*In  this  and  in  all  the  subsequent  experiments,  as  in  those  pre- 
ceding, the  treatment  was  carried  out  in  the  cold,  and  the  solution  used 
contained  the  normal  amount  (20  per  cent)  of  salt,  unless  otherwise 
stated. 

44 


EXPERIMENT  XVI 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Tailings   (i%  K2S,O8)  .3  9.0 

Tailings   (2%  K2S2O8)  .27  4.8 

Whence  it  is  clearly  seen  that  in  reality  the  concentration 
of  the  persulphate  affects  principally  the  oxidation  of  the  silver 
sulphides. 

From  these  various  experiments,  then,  it  becomes  unmis- 
takably evident  that  the  best  conditions  of  operation  in  treat- 
ing such  ores  would  be  to  use  higher  concentrations  of  both 
acids  and  of  persulphate  as  far  as  practicable,  say  5%  of  the 
former  and  2%  of  the  latter,  since  solutions  containing  larger 
amounts  of  the  one  would  be  inconvenient  and  uneconomical, 
while  with  the  other  difficult  to  obtain,  K2Sv,O8  being  only 
slightly  soluble  in  aquous  media. 

In  order  to  determine  how  favorably  the  present  method 
would  compare,  under  such  average  conditions  as  to  the  con- 
centration of  the  acid  and  persulphate  in  solution,  with  the 
chloridation  method  in  extracting  the  precious  metals  from 
the  ore  under  consideration,  another  experiment  was  now 
undertaken. 

Experiment  XVII. — In  this  test  the  concentration  of  the 
solution  in  acid  and  persulphate  was  5%  H2SO4  and  2% 
K2S2O8  respectively,  wjth  the  usual  percentage  of  common 
salt.  Since  at  no  time  during  the  treatment  with  such  a  per- 
centage of  acid  in  solution  would  there  be  the  large  excess  of 
chlorine  on  hand  which  was  the  case  with  the  chloridation  test 
of  the  same  ore  as  previously  described,  it  was  thought  best 
to  equalize  the  conditions  by  having  recourse  to  a  continuous 
percolation  of  the  solution  through  the  ore  mass,  as  would  be 
the  case  in  actual,  large-scale  practice.  To  do  this,  the  ore 
(200  grams)  was  put  in  a  cylindrical  funnel,  the  lower  (inner) 
orifice  of  which  was  covered  with  a  small  piece  of  filter  paper, 
and  the  solution  was  poured  on  the  mass  and  let'  leach  gradu- 
ally until  one  charge  had  entirely  disappeared.  When  this 
was  done,  a  lot  of  fresh  solution  of  the  same  percentage  com- 
position as  the  above  was  poured  on  the  mass  and  again  al- 
lowed to  leach,  the  charge  of  solution  being  then  renewed 

45 


as  before,  and  so  on.  It  must  be  noted  here,  however,  that 
the  first  lot  of  solution  passing  through  the  ore  mass  having 
been  analyzed  and  found  to  have  extracted  iron  from  the  ore 
to  the  extent  of  .4%,  all  subsequent,  newer  additions  of  solu- 
tions were  treated  with  the  corresponding  amount  of  FeCl3, 
so  as  to  render  the  conditions  more  nearly  as  they  would  be 
in  actual  practice. 

When  ten  such  additions  of  solution  and  thorough  perco- 
lation of  same  were  completed,  which  required  about  a  week 
in  all,  the  last  teachings  were  tested  for  silver,  and  they  being 
found  free  of  this  metal,  the  extraction  was  called  complete. 
Fresh  water  was  now  added  to  the  mass  to  displace  the  saline 
solutions  impregnating  the  ore  before  assaying  it.*  This  done, 
the  ore  was  taken  out,  dried,  and  assayed,  giving  the  follow- 
ing results: 

EXPERIMENT  xvn 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Original  .55  49.45 

Tailings  .04  2.7 

These  correspond  to  an  extraction  of  about :  94.6%  silver 

and  92.7%  gold. 

These  results  make  it  clear,  therefore,  that,  as  compared 
with  the  chloridation  method — at  least  as  applied  to  this  par- 
ticular ore — the  persulphatation  method  is  far  more  efficient. 

Having  obtained  such  encouraging  results  with  this  ore,  it 
was  thought  of  interest  to  extend  this  study  to  richer  and 
more  exceptional  ores. 

ORE    NO.     I 

Experiment  XVIII. — 300  grams  of  ore  No.  I,  described  on 
a  former  page,  was  taken,  and,  after  attacking  it  first  with  a 
simple  solution  of  H2SO4  to  dissolve  the  metallic  iron  contained 


*It  is  of  great  importance,  from  the  practical  standpoint,  to  note 
here  that  when  a  weight  of  solution  one-half  that  of  the  ore  used  was 
collected,  it  was  noticed  that  the  teachings  now  became  remarkabjy 
free  from  acid  or  salt,  so  much  so,  indeed,  that  silver  nitrate  would 
give  but  a  slight  turbidity  with  the  percolating  liquid,  while  tasting 
could  not  at  all  detect  the  presence  of  any  saline  matter  in  solution. 

46 


in  it,  enough  of  K2S2O8  was  added  to  oxidize  completely  the 
ferrous  sulphate  thus  formed  to  the  ferric  condition,  which 
done,  common  salt,  more  H2SO4  and  K2S2O8  were  added  to 
the  resulting  solution,  so  that  the  latter  now  contained  20%' 
NaCl,  10%  H2SO4,  and  2%  K,S2O8.  The  experiment  was 
carried  out  exactly  as  was  done  with  the  first  persulphatation 
tests  with  ore  No.  2  (page  42),  that  is,  the  ore  was  treated 
first  for  24  hours,  then  it  was  filtered,  washed,  dried,  and 
assayed  as  usual,  after  which  it  was  again  treated  for  a  sec- 
ond 24  hours  and  assayed,  and  then  treated  for  a  third  24 
hours  and  assayed  for  a  third  and  last  time. 

The  following  is  a  statement  of  the  results  obtained): 

EXPERIMENT  xvm 

An.  Ag.         Per  cent  Extr. 

Sample  oz.  per  ton.  oz.  per  ton.  Gold.  Silver. 

Original  1.03  86.2 

Tailings  after   ist  24  hrs.'   treatment     .66  32.00          36.0      62.9 

Tailings  after  a  2nd  24  hrs'.  treatment     .4  13.2  61.2      84.7 

Tailings  after  a  3d  24  hrs'.  treatment     .4  7.5  61.2      91.3 

Experiment  XIX. — These  results  not  being  quite  satisfac- 
tory, a  second  set  of  experiments  was  undertaken,  this  time 
working  with  three  separate  lots  of  the  ore.  These  were 
treated  with  a  solution  and  in  a  manner  identical  to  the  above, 
only  one  being  assayed  at  the  end  of  the  first  24  hours,  a  sec- 
ond at  the  end  of  48  hours,  and  the  third  at  the  end  of  72 
hours,  the  solutions,  with  the  last  two,  being  merely  decanted 
off  and  renewed  once  every  24  hours,  instead  of  filtering, 
drying,  and  re-treating  them,  as  was  done  in  the  preceding 
experiment.  The  results  now  obtained  were  as  follows : 

EXPERIMENT  xix  • 

Au.  Ag.        Per  cent  Extr. 

Sample  oz.  per  ton.  oz.  per  ton.  Gold.  Silver. 

Original  1.03  86.2 

ist  tailings    (24  hrs'.   treatment)  .57  34.5  44.7      60.0 

2nd  tailings   (48  hrs'.  treatment)  31.2  63.8 

3d  tailings  (72  hrs'.  treatment)  .4  25.5  61.2      70.7 

Leaving  the  low  figures  of  the  first  and  second  tests  aside, 
that  the  third  test,  lasting  three  days,  should  also  thus  fall  so 
far  below  the  degree  of  extraction  previously  obtained  with 
the  chloridation  method  (page  24)  with  this  same  ore  becomes 
now  quite  unaccountable.  The  difficulty  grew  more  mysteri- 

47 


cms,  indeed,  as  it  was  especially  noticed  during  the  lapse  of 
the  experiment  that  in  all  these  tests  the  solutions  were  always 
kept  well  oxidized,  as  was  evidenced  by  the  weak,  but  dis- 
tinct, odor  of  chlorine  which  evolved  from  them. 

Experiment  XX. — Thinking  that  matters  might  perhaps 
improve  if  the  solution  were  left  in  contact  with  the  ore  during 
the  three  days  without  changing  it,  or  again,  perhaps  by  pro- 
longing the  time  of  treatment,  two  more  tests  were  carried 
out  under  these  new  conditions,  keeping  the  percentage  com- 
position of  the  solutions  used  as  that  in  the  preceding  experi- 
ments. These,  however,  gave  no  better  results,  as  can  be  seen 
from  the  following  data: 

EXPERIMENT  xx 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Tailings   (3  days'  treatment)         .6  2I-75 

Tailings  (4  days'  treatment)  .5  21.00 

In  order  to  see  how  far  the  use  of  HC1,  instead  of  H2SO4, 
would  improve  the  results,  another  experiment  was  now  under- 
taken, which  was  also  prolonged  for  4  days,  with  the  follow- 
ing results: 

EXPERIMENT    XXI 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Tailings  (with  HC1,  4  days)  .6  14.7 

These  results  were  certainly  not  very  much  better,  as  they 
still  left  the  high  extractions  obtained  with  the  Chloridation 
method  unattained. 

Experiment  XXII. — It  was  finally  thought  that  perhaps  a 
treatment  under  the  conditions  of  Experiment  XVII  (page 
46),  namely,  resorting  to  a  continuous  percolation  of  the  solu- 
tion through  the  ore  mass  for  a  long  period,  would  give  better 
results.  A  new  test  was  therefore  started  under  these  condi- 
tions, using  a  solution  of  about  the  same  percentage  composi- 
tion, in  every  respect,  as  that  of  the  preceding  experiments, 
the  total  duration  of  the  treatment  being  7  days.  Unfor- 
tunately, however,  even  the  results  thus  arrived  at  were  still 
far  from  being  satisfactory,  as  can  be  seen  from  the  following: 

48 


EXPERIMENT  xxn 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Tailings  (by  percolation)  .33  14.1 

Having  thus  studied  all  the  various  conditions  as  to  the 
concentration  of  the  reagents  in  the  solution,  the  modes  and 
the  durations  of  treatment,  with  a  view  to  attaining  more  satis- 
factory results,  and  having  thereby  ascertained  that  none  of 
these  could  have  been  responsible  for  the  low  extractions  ob- 
tained by  the  present  method,  as  against  those  obtained  by 
chloridation,  it  was  now  thought  that  perhaps  the  sole  expla- 
nation of  the  difficulty  might  lie  in  the  relative  abundance 
of  chlorine  held  in  solution  in  these  two  methods.  It  is  indeed 
a  matter  of  fact  that  while  in  the  experiments  with  ferric  salts 
alone  there  was  always  used  a  large  abundance  of  chlorine 
in  solution,  in  the  present  tests,  on  the  other  hand,  there  was, 
comparatively  speaking,  only  a  small  excess  of  chlorine  on 
hand  at  any  time.  Now  since  when  sulphides  are  decomposed 
and  oxidized  by  means  of  chlorine,  the  oxidation  of  the  sulphur 
in  combination  takes  place  more  completely,  resulting  in  the 
formation  of  H,,SO4,  when  a  large  excess  of  this  gas  is  on 
hand,  whilst  it  only  results  in  the  separation  of  spongy  sul- 
phur when  the  chlorine  on  hand  is  not  sufficiently  abundant, 
it  naturally  seemed  that  the  low  extraction  of  the  precious 
metals  with  the  persulphatation  method,  as  applied  to  this 
ore,  may  have  perhaps  been  due  to  the  formation  of  notable 
amounts  of  just  such  spongy  sulphur,  which  might  then  have 
coated  the  particles  of  the  sulphides  in  the  ore,  and  thus  pre- 
vented their  further  decomposition. 

Now,  if  this  were  so,  it  is  clear  that  the  question  would  be 
to  remove  this  sulphur  by  some  appropriate  method,  and  in 
some  appropriate  manner,  whether  by  a  complete  oxidation 
or  by  a  mere  dissolution  and  removal  of  this  element  from 
the  ore  mass. 

However,  as  the  complete  oxidation  of  sulphur  to  sulphuric 
acid  could  best  be  brought  about,  from  the  practical  stand- 
point, by  the  introduction  of  sufficiently  large  excesses  of 
chlorine  gas  into  the  solution,  and  as  even  such  a  procedure 
could  not  always  be  brought  about  in  large-scale  work,  it  be- 
comes clearly  desirable  to  adopt  some  lixiviation  method 

4Q 


whereby  the  sulphur  may  be  removed  from  the  ore  mass  by 
the  use  of  a  simple,  convenient  solvent  of  that  element,  and 
subsequently  got  rid  of  or  recovered  therefrom  as  far  as 
possible. 

Now,  of  all  the  simple  solvents  of  sulphur,  the  one  which 
would  lend  itself  best  to  this  purpose  is  certainly  a  solution 
of  a  fixed  alkali,  such  as  NaOH,  and  for  good  reasons.  In- 
deed, not  only  could  such  a  solution  be  economically  made  by 
a  simple  electrolysis  of  a  sulphate,  say  Na2SO4,  in  a  diaphragm 
cell,  but,  what  is  of  special  value,  after  the  formation  of  the 
desired  amount  and  concentration  of  the  free  alkali,  and  the 
subsequent  use  of  the  latter  to  dissolve  the  sulphur  in  ores  as 
desired,  the  impure,  sulphuretted  alkaline  solution  thus  result- 
ing could  then  be  treated  with  the  acid  solution  which  had 
formed  and  accumulated  in  the  anode  compartment  of  the 
electrolytic  cell  during  the  electrolysis  of  the  Na2SO4,  and 
thus  made  to  yield  the  sulphur  it  had  extracted  from  the  ore 
either  as  H2S  or  as  free,  precipitated  sulphur,  and  at  the  same 
time  to  regenerate  the  original  Na2SO4,  which  can  then  be 
electrolyzed  again  and  used  as  desired. 

How  well  would  such  alkaline  solutions  answer  for  the  pur- 
pose on  hand? 

Experiment  XXIII. — In  order  to  determine  this  point,  the 
first  tailings  (3-day  treatment)  of  Experiment  XX  were  taken 
up,  treated  with  a  10%  NaOH  solution,  and  left  over  night. 
The  following  day  the  mass  of  ore  and  solution  was  filtered 
off  and  washed  with  pure  water  till  it  was  free  from  alkali. 
This  done,  it  was  now  treated  with  the  ordinary  stock  solution 
of  acid,  salt,  and  2%  K2S2O8,  and  left  stand  2  days  in  the 
cold,  with  occasional  shaking. 

At  the  expiration  of  this  period,  the  mass  was  filtered, 
washed,  dried,  and  assayed,  giving  the  following  results. 

EXPERIMENT  XXIII 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Original  ore  1.03  86.2 

Original  tailings  (Exp.  XX)          .6  21.75 
Tailings  after  second  treatment, 

subsequent    to    the    treatment 

with  NaOH  .11  3.00 

So 


Which  corresponds  to  an  extraction  of  about :     96.9%  silver 

89.3%  gold. 

It  thus  becomes  strikingly  evident  that  the  use  of  NaOH 
in  facilitating  the  further  attack  of  an  ore  by  oxidizing  agents 
is  of  a  great  and  undeniable  value.  On  the  other  hand,  that 
it  is  no  less  undeniable  that  the  action  of  NaOH  in  bringing 
about  this  remarkable  condition  of  affairs  is  in  reality  due  to 
its  power  to  merely  dissolve  considerable  amounts  of  spongy 
sulphur  and  help  to  remove  it  out  of  the  ore  mass,  thus  laying 
bare  the  remaining  particles  of  sulphides  subject  to  the  further 
attack  of  the  main  oxidizing  solution,  is  proven  by  the  fact 
that  the  now  impure,  brownish  solution  of  NaOH,  recovered 
after  the  treatment  of  the  ore  with  it  and  filtration  of  same, 
gave  a  decided  precipitate  of  yellow,  flocculent  sulphur,  when 
neutralized  with  H2SO4,  along  with  a  little  H2S. 

In  fine,  then,  it  is  evident  that  even  such  extreme  ores  as 
the  one  under  consideration  may  be  treated  by  the  use  of  a 
persulphate  alone,  irrespective  of  the  introduction  of  any  chlo- 
rine gas  from  without,  provided  the  use  of  an  alkaline  solution 
is  resorted  to.* 

EXPERIMENTS  WITH  OTHER  ORES 

Besides  the  above,  two  new  samples  of  high-grade  ores 
were  treated  similarly  so  as  to  give  a  rigorous  test  to  the 
method  here  advanced. 

Ore  No.  5. — This  ore  was  obtained  from  Mexico,  and  rep- 
resented a  specimen  sample  of  very  high  grade,  consisting 
chiefly  of  the  simple  sulphide  of  silver  or  Argentite  (Ag2S), 
a  little  ruby  silver  (Ag3SbS4),  gold  (not  free  milling),  and 
a  gangue  of  quartz,  with  some  chalco-pyrite  disseminated 
through  it. 

Experiment  XXIV. — In  order  to  form  an  idea  as  to  the 
degree  of  extraction  which  could  be  obtained  with  coarser 
crushing,  the  ore  was  first  ground  to  5O-mesh  only.  The  test 
was  carried  out  on  a  2OO-gram  lot  by  the  continuous  percolation 
method,  as  previously  described,  using  a  solution  made  up 
of  12%.  HC1,  i%  FeCl3,  and  2%  K2S2O8,  but  with  no  NaCl 


*It  is  scarcely  necessary  to  note  here  that  this  same  modification 
in  the  method  of  ore  extraction  may  be  introduced  also,  if  necessary, 
in  connection  with  the  chloridation  method,  described  in  Part  I  of  this 
paper. 

51 


whatever,  as  it  was  intended  to  have  the  large  percentage  of 
HC1  used  take  its  place. 

The  ore  being  unusually  rich  in  silver,  the  complete  removal 
of  this  metal  required  fully  15  days,  when  the  final  washing 
with  pure  water  was  given  and  the  mass  dried  and  assayed. 
The  following  were  the  results  obtained : 

EXPERIMENT   XXIV 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Original  3.00  680.00 

Tailings  (5O-mesh)  1.32  102.00 

Corresponding  to  an  extraction  of  aboutf:     85%  silver 

and     57%  gold. 

Experiment  XXV. — In  order  to  ascertain  to  what  extent 
finer  crushing  would  help  matters,  these  tailings  were  now 
ground  down  to  70  meshes,  and  again  percolated  with  the 
solution. 

Although  this  new  treatment  showed  some  more  extraction 
of  values,  as  could  be  seen  by  a  dilution  of  the  percolates,  yet 
as  the  effect  stopped  being  noticeable  within  less  than  even  24 
hours,  and  in  a  manner  which  could  not  well  be  taken  to  mean 
that  all  the  remaining  102  ounces  of  silver  per  ton,  for  in- 
stance, had  so  soon  been  extracted  from  the  mass,  it  was 
then  washed  with  water,  and  a  10%  solution  of  NaOH  was 
next  allowed  to  percolate  through  it  so  as  to  dissolve  the  sul- 
phur, which  must  have  accumulated  in  the  ore,  and  thus  to 
free  the  particles  of  .the  ore  from  this  encumbrance  and  facili- 
tate their  further  decomposition  by  the  solution  of  persulphate. 
This  done,  the  ore  was  again  washed  with  water,  and  then 
treated  anew  with  the  stock  solution,  when  a  large  abundance 
of  silver  and  gold  was  immediately  observed  to  have  gone 
into  solution  again.  When  this  action  (lasting  over  3  days) 
was  also  over,  the  ore  was  washed,  dried,  and  assayed,  as  usual, 
with  the  following  results: 

EXPERIMENT  xxv 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Tailings,  after  second  treatment 

(subsequent  to  treatment  with 

NaOH)  .72  34.8 

52 


These  correspond  to  an  extraction  of  about:  95%  silver 

76%  gold. 

Although,  as  it  is  here  seen,  the  silver  extraction  was  very 
satisfactory  considering  the  extremely  rich  character  of  the 
original  ore,  there  was  still  room  for  improvement  as  regards 
the  extraction  of  the  gold  values. 

Experiment  XXVI. — A  third  experiment  was  now  under- 
taken, using  the  same  solution  as  in  the  above  tests  and  with 
the  above  tailings  crushed  down  to  100  meshes.  The  results 
arrived  at  were: 

EXPERIMENT  XXVI 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Tailings    (third   treatment)  .48  24.0 

Corresponding  to  an  extraction  of  about:     96.5%  silver 

and     84%  gold. 

No  further  experiments  were  carried  out  with  this  ore,  as 
a  further  extraction  of  the  precious  metals  was  now  becoming 
increasingly  difficult. 

Ore  No.  6. — This  ore  was  an  average  one,  also  obtained 
from  Mexico,  consisting  chiefly  of  argentite,  galena,  chalco- 
pyrite,  and  some  gold,  with  a  gangue  of  quartz. 

Experiment  XXVII. — Knowing  it  to  be  an  ore  of  the  same 
type  as  the  preceding  one,  this  sample  was  crushed  to  70 
meshes  to  start  with,  and  the  experiment  carried  out  under  the 
same  conditions,  in  all  respects,  as  with  the  above  ore. 

Owing  to  its  being  more  of  an  average  type,  the  extraction 
of  this  ore  was  complete  within  a  week  (3  days  prior  to  treat- 
ment with  NaOH  and  3  days  subsequent  to  it),  when  an  assay 
gave  the  following  results : 

EXPERIMENT  xxvn 

Au.  Ag. 

Sample  Oz.  per  ton.  Oz.  per  ton. 

Original  1.9  106.00 

Tailings  .5  5.3 

Which  corresponds  to  an  extraction  of  about:     95%  silver 

and     73%  gold. 

Although  the  extraction  of  gold  is  still  thus  seen  to  leave 
much  to  be  desired,  no  attempts  were  made  to  repeat  the  ex- 

53 


periment  with  a  finer  crushing  of  the  ore,  as  it  was  rather 
certain  that  better  results  could  thus  be  obtained,  seeing  that 
the  ore  represented,  as  was  alluded  to,  the  same  type  of  ore 
as  the  preceding  one. 

EXPERIMENTS     WITH     ALKALINE     ORES 

There  remains  now  to  consider  the  treatment  of  alkaline 
ores  by  means  of  a  persulphate  in  neutral  solutions,  so  as  to 
determine  to  what  extent  such  ores  may  be  amenable  to  a 
treatment  without  necessitating  the  preliminary  neutralization 
of  their  carbonates  with  correspondingly  large  amounts  of  acids. 

Ore  No.  I. — As  ore  No.  I,  previously  described,  contained 
a  large  percentage  of  CaCo3,  corresponding  to  as  much  as 
15%  H2SO4,  it  was  thought  proper  to  study  it  in  this  con- 
nection first. 

One  hundred  grams  of  this  ore,  powdered,  as  in  all  the  pre- 
ceding experiments  with  the  same  ore,  to  I2omesh,  was  put 
in  a  percolating  cylinder  and  the  mass  treated  with  a  solution 
of  20%  NaCl  and  2%  K2S2O8  alone. 

After  10  days  of  continuous  percolation,  necessitating  ten 
renewals  of  solution,  the  mass  was  leached  with  pure  water, 
dried,  and  assayed,  giving  the  following  results : 

EXPERIMENT  XXVIII 

Au.  Ag. 

Sample  Qz.  per  ton.  Oz.  per  ton. 

Original  1.03  86.2 

Tailings  .52  7.4 

Which  corresponds  to  an  extraction  of  about :     91.4%  silver 

and     49.6%  gold. 

From  these  results  it  will  be  seen  that,  as  might  have  been 
expected,  the  silver  values  of  the  ore  are  satisfactorily  extracted 
by  such  a  treatment  with  a  neutral  solution  of  potassium  per- 
sulphate, although  the  extraction  of  the  gold  values  leaves  still 
much  to  be  desired.  Was  this  latter  due  to  the  extremely  fine 
state  of  division  of  this  metal  in  the  ore,  or  was  it  merely  due 
to  the  extreme  slowness  of  the  liberation  of  chlorine  in  such  a 
neutral  solution  of  a  persulphate? 

54 


Unfortunately,  as  the  writer  has  been  unable  to  obtain  other 
alkaline,  but  fairly  good,  ores  to  test  by  this  modified  method, 
he  is  not  in  a  position  to  answer  the  foregoing  question  defi- 
nitely. It  may,  nevertheless,  be  well  to  note  here  that  ore 
No.  4,  which  was  almost  as  rich  in  carbonates  as  the  above  ore, 
was  also  thus  treated  and  found  to  give  an  extraction  of  about 
the  same  order  as  when  treated  with  the  Chloridation  method,  a 
fact  which,  considering  the  inherently  difficult  character  and 
extremely  low  grade  of  the  ore,  would  seem  to  speak  quite 
favorably  of  the  feasibility  of  economically  treating  certain 
excessively  alkaline  ores  of  the  precious  metals  by  means  of 
a  neutral  solution  of  a  mixture  of  salt  and  a  persulphate,  as 
against  their  treatment  by  the  chloridation  method,  which 
would  necessarily  require  a  preliminary  treatment  of  such 
ores  with  an  acid,  a  necessity  which  might  not  always  be 
trifling. 


ELECTROLYSIS 

So  far  as  the  electrolytic  recovery  of  the  precious  metals 
from  these  persulphate  solutions  is  concerned,  no  detailed  men- 
tion need  be  made  here  of  the  various  steps  to  be  followed 
in  this  operation,  as  it  is,  in  all  essential  details,  identical  with 
that  mentioned  in  the  first  part  of  this  paper  in  connection 
with  the  chloridation  method. 

As  regards  the  electrolytic  regeneration  of  the  main  reagent, 
the  persulphate,  however,  things  are  unfortunately  not  quite 
as  simple.  Indeed,  the  essential  conditions  for  the  electrolytic 
formation  of  persulphates  being  principally  dependent  upon 
the  concentration  of  SO4  ions  transferred  to  the  anodes  (plat- 
inum) per  unit  of  time,  the  unavoidable  introduction  of  large 
amounts  of  a  non-sulphate,  such  as  NaCl  or  HC1,  into  the 
solution  is  quite  incongruous  with  the  purpose  at  issue.  For, 
as  has  been  mentioned  on  a  former  page,  though  a  little  amount 
of  a  chloride  is  highly  favorable  to  persulphate  formation,  yet 
when  higher  concentrations  of  such  a  non-sulphate  salt  are 
used,  the  yield  of  the  electric  current  in  persulphate  formation 
decreases  very  rapidly,  owing  chiefly  to  the  fact  that  these 
foreign  compounds  also  take  part  in  the  transfer  of  electricity 
through  the  solution,  thus  decreasing  the  rate  of  discharge 

55 


of  SO4  ions  at  the  anodes,  and  consequently  the  yield  to  the 
current  in  persulphates. 

In  view  of  this  fact,  it  becomes  apparent  that,  in  order  to 
obtain  satisfactory  yields  in  the  formation  of  persulphates, 
either  the  current  density  at  the  anodes  must  be  increased 
proportionately;  or  some  method  must  be  resorted  to  whereby 
the  bulk  of  the  chlorides  may  be  removed  from  the  solution 
before  attempting  to  regenerate,  or  rather,  reform  the  per- 
sulphate consumed  during  the  treatment  of  the  ore ;  or,  again, 
one  must  strive  to  separate  some  of  the  sulphate  in  solution 
by  some  appropriate  means,  convert  it  into  a  persulphate,  and 
then  return  it  to  the  main  solutions. 

Now  a  consideration  of  these  possible  methods  of  arriving 
at  the  goal  set  forth  will  show  that  the  first  method,  or  that 
depending  upon  the  increase  of  anodic  current  density  with 
solutions  of  a  mixture  of  sulphates  and  chlorides,  is  not  the 
best  that  could  be  desired,  from  the  practical  standpoint,  and 
for  two  reasons.  First,  this  is  so  because  the  current  densities 
used  in  ordinary  practice,  even  with  pure  sulphate  solutions 
(namely,  50  to  500  amperes  per  square  dm.),  requiring,  as 
they  do,  voltages  not  below  5  volts,  sometimes  even  as  high 
as  7  or  8  volts,*  any  further  increase  of  the  current  densities, 
which  would  be  necessary  to  offset  the  injurious  effect  of  the 
presence  of  chlorides,  would  perhaps  mean  doubling  or  even 
trebling  the  latter  figures  for  the  potentials  required  for  the 
purpose  on  hand,  thus  resulting  in  the  decrease  of  the  rende- 
ment  in  persulphates  per  unit  of  power.  .  In  the  second  place, 
the  regeneration  of  the  persulphates  needed  by  direct  electrol- 
ysis of  a  solution  of  sulphates  in  presence  of  much  chlorides 
is  not  desirable,  because,  the  only  metal  admissible  to  serve 
as  anodes  for  such  a  purpose  being  the  costly  platinum,  and 
this  metal  being  far  from  unattacked  when  used  as  anodes 
in  chloride  solutions,  an  economic  difficulty  thus  looms  up 
in  the  way  of  any  continued  practical  operations  in  such  a  line 
of  work. 

Further,  owing  to  the  fact  that  no  special,  economical 
method  can  be  devised  whereby  the  dissolved  chlorides  could 
be  adequately  removed  from  at  least  a  part  of  the  stock  solu- 


*Friedberger  and  Muller,  Ztsch.  fur  Blektrochemie,  '02,  p.  231. 

56 


tion  before  the  electrolysis  of  this  latter  is  carried  qut  in  at- 
tempting to  reform  the  persulphate  consumed,  this  second 
method  must  also  be  considered  impracticable,  if  not  impossible, 
for  large-scale  practice. 

This  leaves  only  the  third  method  above  suggested  as  a 
means  of  regenerating  the  necessary  persulphate,  namely, 
that  consisting  in  the  removal  of  part  of  the  sulphate  in  solu- 
tion from  an  aliquot  part  of  the  latter,  converting  it  into  a 
persulphate  in  a  special  electrolytic  cell,  and  finally  returning 
it  to  the  main  solution  as  needed. 

A  careful  consideration  of  this  possibility  will  show  that 
not  only  could  such  a  method  be  the  most  rational  and  feasible, 
but  that  the  question  thus  viewed  admits  of  more  than  one 
solution.  Indeed,  since  the  problem  is  one  of  separating  a 
part  of  the  sulphate  from  the  stock  solutions  by  crystallization, 
it  is  clear  that  this  may  be  brought  about  in  two  ways,  to  wit : 
either  by  concentrating,  by  evaporation,  an  aliquot  part  of 
the  solution  until  the  sulphate  alone  begins  to  separate — the 
original  solution  being  more  nearly  saturated  with  respect 
to  this  salt  than  with  respect  to  the  chloride  in  solution — or 
by  taking  advantage  of  the  decrease  in  the  solubility  of  certain 
sulphates  by  lowering  the  temperature  of  their  solutions. 

Thus,  if  the  sulphate  worked  with  in  practice  be  the  potas- 
sium salt,*  part  of  the  liquors  holding  this  latter  in  solution 
may  be  shunted  off — best  directly  after  the  recovery  of  the 
precious  metals  therefrom — and  a  desired  amount  of  the  sul- 
phate made  to  separate  out  in  the  solid  form,  either  by  con- 
centrating the  solution  with  respect  to  this  salt  by  evaporation, 
or  by  cooling  it  sufficiently,  according  to  practical  facilities. 
In  either  case,  the  solution  being  more  nearly  saturated  with 
respect  to  the  sulphate  than  the  chloride  present,  the  former 
salt  will  naturally  separate  out  first,  in  form  sufficiently  pure 
and  free  from  admixed  chlorides  to  be  redissolved  in  a  fresh, 
sufficiently  chloride-fre'e  solution  of  the  same  sulphate,  and 
electrolyzed  in  special  electrolytic  cells,  with  a  view  to  convert- 
ing it,  at  least  in  part,  to  a  persulphate,  which  may  then  be 
removed  from  the  solution  and  returned  to  the  stock  liquors. 

While,  with  potassium  sulphate  chosen  as  the  starting 
material  to  work  with  in  practice,  the  evaporation  method 

*Same  would  be  true  with  (NH4)2SO4. 

57 


seems  to  be  much  more  likely  to  be  practicable,  with  the 
sodium  salt  (Na2SO4),  on  the  other  hand,  the  separation  of 
the  solid  Na2SO4  from  the  shunted  solutions  by  cooling  seems 
to  be  equally  likely  to  meet  with  success  in  practical  operations. 
Indeed,  the  very  rapid  decrease  of  solubility  of  the  Na2SO4 
with  the  lowering  of  the  temperature  of  its  solutions,  and  the 
very  slight  decrease  under  the  same  conditions  of  the  solu- 
bility of  NaCl  which  may  be  present  in  the  same  solution, 
render  this  method  highly  useful  as  a  means  of  bringing  about 
the  separation  of  the  former  salt  from  a  concentrated  solution 
of  both  these  compounds  in  a  form  sufficiently  free  from  the 
latter  for  all  purposes  concerned,  and  that  with  only  a  slight 
lowering  of  the  temperature  of  the  solution. 

But  to  what  extent  can  this  happy  coincidence  be  turned 
into  practical  use,  supposing  Na2SO4  to  be  chosen  as  the 
starting  material  to  be  converted  into  the  corresponding  per- 
sulphate? Would  not  the  difficulty  of  preparing  the  persul- 
phate of  sodium  in  solid  form — owing  to  its  excessive  solu- 
bility— stand  in  the  way  of  the  sulphate  of  this  element  being 
used,  in  actual  commercial  practice,  as  the  starting  material 
needed  ? 

While  not  prepared  to  make  light  of  this  difficulty,  the  writer 
is  of  the  opinion  that  it  is  not  absolutely  necessary  to  prepare 
this  persulphate  in  the  solid  form,  but  that  the  sulphate  of 
sodium  (whether  original  or  recovered  from  the  shunted  parts 
of  the  stock  solutions)  may  be  dissolved  in  water  and  then 
converted,  to  any  desired  extent,  into  a  solution  of  the  corre- 
sponding persulphate,  which  solution  may  then  be  returned, 
in  body,  to  the  main  or  stock  liquids.  Nor  is  it,  perhaps,  true 
that  sodium  persulphate  cannot  be  obtained  in  the  solid  form, 
although  all  authentic  accounts  of  the  authorities  who  have 
dealt  with  the  subject  from  a  purely  scientific  standpoint  either 
seem  to  categorically  deny  its  feasibility,*  or  at  least  make  no 
mention  of  any  one  having  met  with  success  in  that  line.  In- 
deed, according  to  Lowenherz,§  the  persulphate  of  sodium  can 
be  obtained  without  any  special  difficulty,  provided  the  current 
density  is  increased  to  unusually  high  limits. 


*Marshall,  /.  Soc.   Chem.  Industry,  '97,  p.  396 ;   Elbs,  Ztsch.   fur 
Elektrochemie,  '95,  p.  245. 

§Lowenherz,   Wagner's  Jahrsbr.   Chem.    Technol,  '95,  p.  366. 

58 


As  the  account  of  Lowenherz's  work  is  got  from  his  patent 
specification,  however,  arid  as  no  other  authority  seems  to 
have  thus  far  proven  it  to  be  false  or  correct,  it  would  be 
unsafe  to  draw  any  foregone  conclusions,  in  positive  terms, 
for  or  against  such  a  possibility.  Personally,  the  writer  re- 
grets that  he  has  had  no  opportunity,  as  yet,  to  verify  or  dis- 
prove the  contentions  of  the  patent.* 

Just  what  should  be  the  exact  form  of  the  cells  and  other 
appliances  to  be  used  in  this  operation  of  regenerating  the 
persulphates  it  would  be  unnecessary  to  discuss  here.  The 
question  belongs,  strictly,  to  the  domain  of  practical,  large- 
scale  operation,  and  could  therefore  be  best  studied  and  decided 
upon  under  such  conditions.  Suffice  it  to  say  in  this  connec- 
tion, however,  that  the  contrivances  and  methods  to  be  used 
in  practice  need  be  neither  complicated,  involved,  nor  costly, 
since  one  has  to  deal  with  only  a  rather  limited  volume  of  so- 
lutions at  a  time,  from  which  to  crystallize  the  needed  amount 
of  the  sulphate,  and  then  to  "persulphate"  the  latter  by  elec- 
trolysis. In  other  words,  these  contrivances  need  not  be  ma- 
terially different  from  those  made  use  of  in  the  ordinary  com- 
mercial method  of  the  preparation  of  persulphates. § 


RESUME 

Two  independent  cyclic  methods  have  been  studied  and  ad- 
vanced, in  the  present  work,  for  the  simultaneous  winning  of 
the  precious  metals  from  their  sulphide  ores. 

The  first,  or  "chloridation,"  process  depends  upon  the  use 
of  acid  solutions  of  ferric  chloride  or  sulphate,  in  presence 
of  high  concentrations  of  soluble  chlorides,  especially  hydro- 
chloric acid  or  sodium  chloride,  which  act  as  solvents  of  silver 
chloride.  With  sodium  chloride  used  as  a  solvent,  it  was 
observed  that,  under  certain  conditions,  an  insoluble  complex 
salt  of  silver  and  sodium  chlorides  would  separate  in  form 


*It  is  important  to  note  here  further  that  besides  the  above  alkali 
sulphates,  that  of  iron,  and  especially  of  aluminum,  could  also  be 
used  in  this  connection  in  converting  them  to  their  corresponding 
persulphates. 

$  See  foregoing  references. 

59 


of  definite  isometric  (cubic)  crystals.  Attempts  were  made  to 
obtain  this  salt  in  condition  sufficiently  free  from  extra  sodium 
chloride  for  analysis,  but  without  success.  This  salt  seems 
to  be  best  represented  by  the  formula  AgCl.NaCl-f xNaCl, 
where  x  represents  the  number  of  molecules  of  extra  sodium 
chloride  which  will  unavoidably  crystallize  isomorphously  with 
the  double  salt  AgCl.NaCl.  This  may  therefore  be  regarded 
as  an  interesting  case  where  double  salt  formation  and  iso- 
morphism may  coexist. 

Various  ores  were  treated  by  this  process,  and  quantitative 
data  obtained  showing  an  average  extraction,  under  proper 
conditions,  of  over  95%.  silver  and  from  60%  to  88%  gold, 
varying  with  the  character  of  the  ore. 

The  metals  dissolved  were  recovered  by  electrolysis.  It 
was  found  that,  under  proper  conditions,  the  reoxidation  of 
the  ferrous  salts  formed  during  the  decomposition  of  the  sul- 
phides, and  the  consequent  regeneration  of  the  ferric  salts, 
could  be  readily  accomplished  without  the  use  of  any  dia- 
phragms whatever.  This  is  of  special  interest,  as  in  all  the 
electro-chemical  processes  heretofore  suggested  for  this  pur- 
pose the  use  of  some  diaphragm  has  been  thought  to  be  an 
unavoidable,  though  troublesome,  necessity.  It  was  further 
found  that  there  was  no  difficulty  in  carrying  the  reoxidation 
of  the  solution  so  far  as  to  even  charge  the  latter  with  much 
free  chlorine,  which  could  then  act  as  an  efficient  solvent  of 
gold. 

A  very  small  percentage  of  glue  added  to  the  solution  was 
found  to  give  good,  coherent  deposits  of  the  metals. 

The  second,  or  "persulphatation,"  method  studied  depends 
upon  the  use  of  persulphates,  also  in  conjunction  with  con- 
centrated solutions  of  chlorides.  Various  experiments  were 
carried  out  to  study  the  effects  of  varying  the  concentration 
of  acids  and  persulphate  in  solution.  The  best  results  were 
obtained  with  about  2%  persulphate  of  potassium  and  5% 
sulphuric  acid.  The  extractions  correspond  to  over  95%  silver 
and  from  70%  to  92%  gold. 

It  was  observed  that  an  intermediate  leaching  of  the  ore 
masses  with  a  solution  of  an  alkali,  such  as  sodium  hydrate, 
would  facilitate  the  further  attack  of  the  sulphides.  This 
was  found  to  be  due  to  the  fact  that  such  alkalies  dissolve 

60 


away  the  sulphur  and  help  to  remove  this  element,  which 
otherwise  forms  a  coating  around  the  particles  of  the  sulphides 
and  thus  hinders  their  further  decomposition.  The  reaction 
giving  rise  to  the  formation  of  free  sulphur  may  be  represented 
thus: 

Ag2S+K2S208  =  Ag2S04+K2S04+S 

One  important  advantage  of  this  second  process  was  found 
to  lie  in  the  fact  that  it  could  be  used  with  ores  that  are  too 
highly  alkaline  with  carbonates  to  admit  of  the  expenses  of  a 
preliminary  neutralization  of  the  ore  masses  with  acids,  which 
would  be  necessary  if  the  first  process  were  to  be  used. 

A  test  has  been  suggested,  in  this  connection,  to  detect  small 
amounts  of  silver  held  in  solution  by  virtue  of  the  presence 
of  chlorides.  This  test  depends  upon  the  use  of  a  persulphate 
in  presence  of  free  ammonium  hydrate,  when,  if  silver  is  pres- 
ent, an  abundant  evolution  of  free  nitrogen  gas  takes  place. 
This  is  due  to  a  catalytic  action  depending  upon  the  inter- 
mediate formation  of  peroxide  of  silver  and  its  immediate 
action  upon  the  free  ammonia  present. 


61 


ACKNOWLEDGEMENTS 

The  writer  wishes  to  extend  here  his  grateful  thanks  to 
Professors  Edmond  O'Neill  and  W.  B.  Rising,  and  to  Dr. 
F.  G.  Cottrell  and  all  the  members  of  the  Faculty  of  this  Depart- 
ment, for  assistance,  advice,  and  kind  considerations  shown 
to  him  while  engaged  in  the  foregoing  studies. 


62 


REC'D  LD 

OCT211957 


YC  68574 


